Structural mechanics is a science which studios the strength, the rigidity and the stability of engineering structures and parts thereof. The strength of materials dealing with the strength, rigidity and stability of isolated members; the theory of elasticity, which is concerned with the same problems but gives more strict solutions; the theory of plasticity which investigates the stresses and strains of plastic and elasto-plastic materials» and finally the theory of structures which studies the strength, rigidity and stability of whole structures—all form parts of this discipline.