Design your own truss.

Calculation of forces in a truss.

The forces (red = tension, blue = compression) in a simple truss with a load of 50 at the center node are shown in the figure. There is a neat web site that lets you design and calculate the forces in trusses that you come up with: Bridge Designer

There are some limitations in the program that restrict the designs but its fun to play with. Early bridge builders did not have the benefit of such design tools and seem to have worked by trial and error. Experience has been called the best teacher however bridge failures were not uncommon.