For a statically indeterminate structure, equations of equilibrium alone are not sufficient to permit analysis (see Art. 3.28). For such systems, additional equations must be derived from requirements ensuring compatibility of deformations. The relationship between stress and strain affects compatibility requirements. In Arts. 3.35 to 3.39, linear elastic behavior is assumed; i.e., in all cases stress is assumed to be directly proportional to strain.
There are two basic approaches for analyzing statically indeterminate structures, force methods and displacement methods. In the force methods, forces are chosen as redundants to satisfy equilibrium. They are determined from compatibility conditions (see Art. 3.35). In the displacement methods, displacements are chosen as redundants to ensure geometric compatibility. They are also determined from equilibrium equations (see Art. 3.36). In both
methods, once the unknown redundants are determined, the structure can be analyzed by
statics.
