"Shell-like structures are familiar enough in nature but the use of such structures as containers, aircraft fuselages, submarine hulls and roofing structures is only of recent origin. That the inherent strength of shells... has not been utilised much in the past is probably due to the difficulty in obtaining suitable material... [S]hell structures in general are these days constructed of such varied materials as steel, light alloy, plastics, wood and reinforced concrete. ...[T]o simplify analysis it will be assumed that the material... is homogeneous, isotropic and perfectly elastic. ...[A]lthough reinforced concrete behaves in a reasonably elastic manner only in the lower stress ranges the majority of reinforced concrete shell roofs that are constructed in practice are designed as elastic structures."