A structural system is not only effected by external conditions, but also by the properties and behavior of the materials which comprise it. These also determine the nature of the system's reaction(s) to external forces. The study of Strength of Materials is concerned specifically with the following issues:
Statics is the study of the behavior of rigid bodies at rest as they are acted upon by external forces. Although most of these bodies are not absolutely rigid, the assumption of rigidity is valid for the purpose of determining the reactions of the system. Actually, every material will deform under a load. Even a concrete slab deforms microscopicly when a person walks on it. Some deformations in a structure can be detrimental to the overall system's performance, while others might only be an issue of comfort. The recognition of the relative importance of these deformations will be an important part of the study of structures.
External loads on a structural system create resisting forces within all of the members that form the load path from the point of the application of the load to the ground beneath the foundation. This internal resistence exists within every member and joint included in the load path and are known simply as the internal forces acting on a member.
Some of these forces have already been examined: the connection between a beam and its support, and the connection of a two-force member to a three-force member in a pin-connected frame. The internal forces within a beam were demonstrated by cutting a beam and drawing a FBD. These internal forces were required at the cut section to put the beam back into equilibrium. The forces and moments that were examined were applied externally to the end of that cut section; they were exactly equal to the internal forces and moments.
The distribution of the internal forces on the cross-sectional area of a member may or may not be uniformly distributed; it is dependent on the loading condition, the type of member, and how it is supported.