Building a truss bridge from toothpicks is a common science class project intended to teach distribution of force, capacity, resilience, strength and basic principles of engineering. Teachers can break classes or groups of students into teams to compete to see who can build the strongest bridge. The most common bridge design used in this exercise is the Warren truss bridge, patented by James Warren and Willoughby Monzoni in 1848. There are many variations on this basic design, such as the Warren Vertical Support truss and the Warren Subdivided truss. Before attempting to create complicated structures, it is best to master the basic Warren design.
Draw a to-scale plan for both sides and the bottom of your bridge. Consult engineering and physics resources before trying to elaborate too much on the basic Warren design. It is essential to include diagonal trusses that point to the midpoint of the bridge. Those diagonal trusses take on the compression and stress of whatever load crosses the bridge, preventing it from collapsing. Draw in reinforcements for the diagonal trusses with multiple toothpicks.
Lay out your toothpicks on top of the design. Carefully glue your toothpicks together (but not to the paper) so that they create two sides and a bottom for your bridge. Allow the glue to dry fully (two to four hours should be adequate).
Examine your bridge pieces, comparing them to examples of a Warren truss in books or online. If you see weak points, reinforce those areas with additional toothpicks. If you plan to subdivide your bridge trusses, add those toothpicks into your design at this point and glue them into place. (Allow those toothpicks to dry, too.)
Assemble the pieces of your bridge with glue. Allow your project to dry overnight before testing the capacity.
Build the base for your bridge following the directions in a base diagram.
Test the capacity of the bridge (using the methods found in Mr. G Online: Toothpick Bridge Project) before entering it in any competition or submitting it for a class project. This will give you the opportunity to examine your design and see its durability and flexibility prior to finishing the project so you can correct design flaws and reinforce weak areas of flex.