Modern thin concrete shells, which began to appear in Europe in the 1920s, are made from steel reinforced concrete of uniform thickness as thin as 2”-4” depending on the span. In many cases there were no supplementary ribs or additional structure, relying wholly on the thin slab or shell to perform the major structural tasks in the building. Modern shells were first introduced by architects and engineers such as Eugène Freyssinet (1879-1962), Bernardo Laffaielle (1900-1955), Pier Luigi Nervi (1891-1979), Eduardo Torroja (1899-1961), Félix Candela (1910-1997), among others. The strongest form of shell is the monolithic shell, which is cast as a single unit. The most common monolithic form is the dome, but ellipsoids and cylinders and variations thereof are also possible.
The design and construction of shell structure were a trend through the ‘60s. However, the approach to this type of structural and architectural design declined due to the high costs of labour, concrete, and cost of the complex project specific formwork. The shells also require a reasonably high level of maintenance to prevent leaks and other construction pathologies due to the exposed concrete also serving as the roof and primary moisture barrier. Since the 1980s, the preference for polygonal shapes and stretched structures occurred.