Recent contributions to the theory of structures with particular reference to the design of aeroplanes have dealt with the theory of frames in which the members are rigidly connected together at the joints; the object being to ascertain the degree of increase in strength of the individual members due to the restraint at their ends supplied by the contiguous members.

Briefly, when a member forms part of a rigid-jointed structure it acquires a rigidity arising from the rest of the structure. The amount of rigidity given by the rest of the structure may vary from zero end fixation to full encastreing.