Bars are usually connected in two or three dimensions to form more complex structures called trusses. Common examples of truss structures are roof trusses and bridges.
A model for a truss is usually idealized according to the figure below.
The bars are assumed to be connected using hinge joints which can only transfer forces along the directs of the bars, i.e. $\alpha=0$ in the figure below. This is a modeling assumption — a simplification of reality — which is not fully accurate.
To see that the forces can only be transmitted along the bars, consider moment equilibrium around any joint. Thus, it is only the magnitude of the sectional force that is unknown, the direction of the force must coincide with the direction of the bar.
Statically determinate trusses
There are two classic methods for analyzing statically determinate trusses: the method of joints and the method of sections. These methods will now be described in short.
Method of joints
The method consists of making a free body diagram around each joint and establishing equilibrium for the join. For each joint one can establish two equations for equilibrium (three equations for a three-dimensional truss). The moment equation does not provide additional information since each sectional force has its line of action through the joint giving a leverage are that is zero.
Method of sections
This method is useful when you are only interested in a few sectional forces and do not want to through joint by joint. For both methods, (method of joints and method of sections), it is easy to make mistakes and if you make one mistake everything that follows will be wrong. It is, therefore, important to check to that the equations of equilibrium are satisfied.