Revolutionary changes in structural design are so rare that particular interest attaches to occasional reports, principally from foreign countries, describing the manufacture in light aluminium alloys of structures which would normally be of steel. Clearly the choice of material is not dictated by some irresponsible whim, but by economic considerations, or other equally strong justification. For many years, the use of aluminium alloys has been standard practice for certain special types of structures, e.g., aircraft, and if proof were necessary of the suitability of aluminium alloys for withstanding stress, fatigue, corrosion, and other factors looked for in a structural material, it would be found in the behaviour of the delicate,
complicated girderwork of a rigid airship, or the less intricate but equally elegant structure forming the fuselage and wings of an aeroplane. For such structures, however, the paramount importance of the stress-weight ratio reduces to a secondary consideration those questions of comparative cost which in general engineering are so important. The fact that the cost of duralumin may be 2s. 6d. per lb., whereas structural steel costs less than 2d., is of less importance than the fact that l-lb. of
duralumin in the form of a bar of any given length will support nearly three times the
load carried by l-lb. of structural steel of the same length.