The main function of the trusses is to support roof, strengthen bridges, or support towers. They are developed for sustaining loads. Generally they are built with long, straight members associated with joints at the end of each member. Jeff Hanson shows the method of joints system in order how to discover a zero force member for trusses.
See the video presenting one of the most recognized methods for finding out the unidentified forces functioning on the individual members of a truss.