A concrete shell, also commonly called thin shell concrete structure, is a structure composed of a relatively thin shell of concrete, usually with no interior columns or exterior buttresses. The shells are most commonly flat plates and domes, but may also take the form of ellipsoids or cylindrical sections, or some combination thereof. The first concrete shell dates back to the second century. The shell structure is typically found in nature as well as in classical architecture. Its efficiency is based on its curvature (single or double), which allows a multiplicity of alternative stress paths and gives the optimum form for transmission of many different load types. Shell structures predominantly resist loads on them by direct compression. That is without bending or flexure. Since most materials are more effective in compression than in bending, shell structures result in lesser thickness than flat structures. A thin shell is defined as a shell with a thickness which is small compared to its other dimensions and in which deformations are not large compared to thickness.
A primary difference between a shell structure and a plate structure is that, in the unstressed state, the shell structure has curvature as opposed to plate’s structures which are flat. Membrane action in a shell is primarily caused by in-plane forces (plane stress), though there may be secondary forces resulting from flexural deformations. Where a flat plate acts similar to a beam with bending and shear stresses, shells are analogous to a cable which resists loads through tensile stresses. Though the ideal thin shell must be capable of developing both tension and compression. Shell structures can usually be understood as a set of beams, arches and catenaries. Man-made shell structures as used in various branches. Engineering structures and/or architectural works whose structure is defined as Thin-shell structures from around the world. Thin shell structures are uniquely suited to carrying distributed loads and find wide application as roof structures in building. Shell structures are widely used in civil, mechanical, architectural, aeronautical, and marine engineering
Various different types of steel shell structures have been used for industrial purposes; singly curved shells, for example, can be found in oil storage tanks, the central part of some pressure vessels, in storage structures such as silos, in industrial chimneys and even in small structures like lighting columns. The single curvature allows a very simple construction process and is very efficient in resisting certain types of loads. In some cases, it is better to take advantage of double curvature. Double curved shells are used to build spherical gas reservoirs, roofs, vehicles, water towers and even hanging roofs. Distributed loads due to internal pressure in storage tanks, pressure vessels or silos, or to external pressure from wind, marine currents and hydrostatic pressures are very well resisted by the in-plane behaviour of shells. Shell structures often need to be strengthened in certain problem areas by local reinforcement. Local reinforcement is also often required at connections between shell structures, such as commonly occur in general piping work and in the offshore industry. In contrast to local reinforcement, global reinforcement is generally used to improve the overall shell behaviour. In axisymmetric shells, the obvious location for the stiffeners is along selected meridians and parallel lines, creating in this way a true mesh which reinforces the pure shell structure.
The development of the design of the Sydney Opera House may probably be regarded as the first major example of a modern concert hall in which the general shapes, plus many smaller details, were radically influenced by thorough scale model testing. Sydney Opera House is a landmark of the city of Sydney. Its unique sail like architecture does not only attract serious architects, it is also eye-catching to curious tourists. This is the only architectural form in the 20thcentury that is classified as world heritage. The revolutionary design of Utzon had many problems that were unique and the solutions had to be generated from scratch. The main one was particular to Utzon’s design. The first problem was the roof.
Though the sail-like roof was eye-catching, the shapes were not defined by regular geometry and the engineers had great difficulty in defining precisely the ‘free-form’ shapes, which led to problems in calculating the loads the shells would have to carry. Utzon proposed the spherical geometry solution. Later he realised that the surface of the regular form of a sphere could be sliced to give the necessary pieces for the shells. The uniformity over a curved surface not only gave the shells well-defined shapes, but also a simple and economic process of prefabrication. This final solution for the shells, however, had an obviously different shape from the original roof design. It is the first example of the use of computer analysis to design complex shapes.