The Arizona Science Lab presents information on types of beams used for building bridges. They show the huge difference in resistance to bending between a flat beam and an I Beam. Then different types of truss designs are shown and explained. Afterwords the students are given supplies to build their own bridges, and this video highlights some of them.
Their goal is to make a bridge which holds 50 pounds over a 2 foot span. Their materials are popsicle sticks and hot glue. Hot glue is very flexible and probably contributed to the premature failure of many of these bridges. You can see in the video how important the loading process is to the strength of the bridge. Hot glue doesn’t not help with the lateral stability of the models, and they are susceptible to just flopping over.