http://www.archdaily.com/11602/shell-artechnic-architects/http://www.ketchum.org/ShellTandF/Structures, which keep their shape and support loads, even without a frame, or solid mass material inside, are called shell structures. These structures use a thin, carefully shaped, outer layer of material, to provide their strength and rigidity. The shape of a shell structure spreads forces throughout the whole structure, which means every part of the structure supports only a small part of the load, giving it its strength. Examples include: igloos, egg cartons, turtle shell, food or pop cans, or, even bubbles in foam and cream puffs.Shell structures are enclosed and hollow in the middle e.g. a snail shell or a house.Frame structures are open on the sides e.g. a tree or the Eiffel tower.shell structure,In building construction, a thin, curved plate structure shaped to transmit applied forces by compressive, tensile, and shear stresses that act in the plane of the surface. They are usually constructed of concrete reinforced with steel mesh (seeshotcrete). Shell construction began in the 1920s; the shell emerged as a major long-span concrete structure after World War II. Thin parabolic shell vaults stiffened with ribs have been built with spans up to about 300 ft (90 m). More complex forms of concrete shells have been made, including hyperbolic paraboloids, or saddle shapes, and intersecting parabolic vaults less than 0.5 in. (1.25 cm) thick. Pioneering thin-shell designers includeFelix CandelaandPier Luigi Nervi.Recent years have seen a renaissance in shell structures. Both through advances in computation and materials, engineers and architects have been imagining and creating beautiful thin shell structures. The design and engineering of shell structures is a highly involved process. Particularly efficient or so-called funicular shell shapes, that carry their self-weight in pure compression, require a process of form finding. Their shape depends on the flow of forces and vice versa. The design of shell structures in general may rely on additional mechanical, technical or architectural aspects necessitating some form of structural optimization.

. The two types of shells have uses which are altogether different and the architectural and engineering problems require a different approach. There are, of course, borderline cases where it is difficult to distinguish between the long an short shell.In structures making use of the short shell, the principle structural element is the stiffener, usually a reinforced concrete arch, although steel arches or trusses have been used. The short shell serves only a minor role, therefore, the emphasis in this chapter then will be on the arch shape. Many structures built with short shells, such a large hangars and auditoriums, could have been built with little more dead load by using a ribbed slab or other lightweight concrete framing system rather than the shell. The architecture of short shells, therefor, must be based on the exploitation of the shape of the arch rather than on the shell itself.BASIC ELEMENTS OF SHORT SHELLSThis sketch illustrates some of the principle parts of a short shell structure: 1) the shell spanning between arches, and 2) the arch structure. In this structure, the edge beams are provided at the lowest point of the shell and the arch is placed on top of the shell so that forms may be moved through the barrel. In small structures, the edge beam can be omitted if the shell is thickened. The curve of the shell is determined by the proper shape of the arch and may be a circle for small structures or may conform to the thrust line of the arch for long span structures.The minimum shell thickness should be at the top in the center of the span. At the arch, the shell thickness is increased slightly for local stresses. The thickness increases toward the springing line of the arch and if not supported by an edge beam, the thickness here should be based on the thickness for a slab spanning the same distance. The edge beams act like the folded plate structures described in the first chapter.

PURE ARCH AND SHELLThe classic simplicity of this structure may be used with startling effect. There are only two structural elements and these are clearly expressed so that their function is evident. Obviously, if the shells are obscured by the walls necessary to enclose this space, much of the effect is lost. However, window walls would be in keeping with the spirit of the design and can be made to follow the curve of the arch.If this structure is to be used as a canopy, the obvious curve of the arch is a ellipse because the arches can spring almost vertically from the ground and the slanting member will not be as great a hazard to people's heads. The curve requiring the least material would be the thrust line, or funicular curve, for the loads on the structure. This form would have considerable curvature at the top but would be practically straight from the edge of the shell to the ground. The larger the arch span, the greater the saving of concrete and reinforcing by the use of a funicular curve.