For a given frame or a truss the support reactions are determined taking moments of the external forces with respect to the support. Then each joint is considered individually as a free body in equilibrium and the forces on the members at that joint are determined by summing up all the vertical component of forces to zero and all the horizontal component of forces to zero.
i.e., ΣV = 0
and ΣH = 0
Joints should be selected such that forces for only two members are unknown in that joint. The force is said to be tensile if it pulls the joint to which it is connected.
This method is explained in detail in the solved problems given below.
Determine the forces in the members ...