Overview
To apply the method of joints to find the forces in a truss's members.
For a truss, the method of joints is used to determine the forces in the members. To find each member's force, the equilibrium of individual joints is considered. The member forces become external forces in a joint's free-body diagram and must satisfy the following equations of equilibrium:
∑Fx=0
∑Fy=0
Because each joint is subjected to a coplanar and concurrent force system, no moment equations are needed. The analysis of a truss should always begin with a joint that has at least one known force and at most two unknown forces. These equations of equilibrium yield two algebraic equations that can be solved for at most two unknowns.