"[T]he basic model of a shell which we shall use... takes as its first step the replacement of a shell by a surface... [M]uch of the detail of the stress distribution will be suppressed precisely in the step of shrinking the three-dimensional physical shell into a zero-thickness surface. ...[T]he 'surface' theory... is extremely simple in comparison with other theories... [T]he regions in which the theory is inadequate are all highly localized, and there are very many practical problems in which... the local details are either unimportant or else can be treated more or less in isolation. All if this is closely analogous to the classical methods for analyzing beam and frame structures."