Three of the primary considerations involved in the design of an aeroplane are safety, weight and serviceaability. The structure weight being a major item in the design which can influence its efficiency as a flying machine, the considerations of safety and serviceability have always to be studied with respect to their effect on the weight of the structure. The history of the development of aircraft structural design indicates clearly the tremendous amout of work which has been devoted to the development of higher strength materials and more accurate stressing and testing methods, always with the aim of ensuring adequate safety and reliability consistent with efficiency of design, construction, and operation.