Recall, for all plane truss structures, the following three conditions
- All members are in the same plane.
- All loading is at the joints.
- All joints are pinned (free rotation).
These conditions require all member forces to act in the direction of the member, and there are no moment loads in the truss members (assuming straight members).
In the previous Method of Joints, each joint was independently analyzed which resulted in numerous joint calculations. If only one member force is desired it does not make sense to solve for many joints. The Method of Sections is an alternative method to solve for interior member forces that is simpler than the Method of Joints.
In the Method of Sections, the truss is cut into two sections. The removed section is replace with unknown member forces acting in the direction of the cut member. They can be pointing in either direction, but generally,
they are drawn away from the section to represent tension.
The unknown member forces at the section cut can be solved using the equilibrium equations, ΣFx = 0, ΣFy = 0, and ΣM = 0.
Since there are only three equilibrium equations, the truss section cut should be located where there are only three unknown member forces.