In stone bridges, the arch is vital. In truss bridges, the important form is the triangle. For thousands of years, crude forms of the truss bridge were built but without knowing how to give a truss the most strength. A square shape can be bent out of shape, as we have seen. It won't hold anything up. Any weight on it could cause it to collapse. A triangle, however, cannot be distorted. You cannot change this basic shape. This is why triangles are so important in construction. Before designing a model bridge that will hold the greatest load, students will learn how to choose the sizes of individual members in the most efficient proportion to the magnitudes and characters of the forces they must carry.
The book Shaping Structures, by Waclaw Zalewski and Edward Allen, has a lot of great information on the effects and distribution of forces on bodies and the art of shaping structures. In the companion CD, there are eight excellent step-by-step models for finding and designing forces in a truss. Skill level for students for this CD program should be at least strong Algebra 1.
What happens when you change the depth of a truss? The Force Polygons show that if you reduce the depth of a truss by half, member forces in the top and bottom chords exactly double! To keep forces low, make the truss as deep as possible. Longer trusses mean more material, however, so a depth has to be chosen that is optimal for given conditions.