CONCRETE SHELLA 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. UsesMost concrete shell structures are buildings, including storage facilities, commercial buildings, and residential homes. Concrete shell construction techniques are well suited for complex curves, are also used to build boat hulls (called ferroconcrete). Civil engineering students often build concrete canoes during their undergraduate classwork in order to explore the techniques involved in building concrete shells. The canoes are then raced in a competition, and in order to be competitive, the canoes must be both lightweight and strong.

ADVANTAGES

Like the arch, the curved shapes often used for concrete shells are naturally strong structures, allowing wide areas to be spanned without the use of internal supports, giving an open, unobstructed interior. The use of concrete as a building material reduces both materials cost and construction costs, as concrete is relatively inexpensive and easily cast into compound curves. The resulting structure may be immensely strong and safe; modern monolithic dome houses, for example, have resisted hurricanes and fires, and are widely considered to be strong enough to withstand even F5 tornadoes. DisadvantagesSince concrete is a porous material, concrete domes often have issues with sealing. If not treated, rainwater can seep through the roof and leak into the interior of the building. On the other hand, the seamless construction of concrete domes prevents air from escaping, and can lead to buildup of condensation on the inside of the shell. Shingling or sealants are common solutions to the problem of exterior moisture, and dehumidifiers can address condensation.

HISTORIC CONCRETE SHELLS

The oldest known concrete shell, the Pantheon in Rome, was completed about AD 125, and is still standing. It has a massive concrete dome 43m in diameter. A monolithic structure, it appears to have been sculpted in place by applying thin layers on top of each other in decreasing diameter. Massively thick at the bottom and thinning at the top, the Pantheon is a remarkable feat of engineering.The Seattle Kingdome was the world's first (and only) concrete-domed multi-purpose stadium. It was completed in 1976 and demolished in 2000. The Kingdome was constructed of triangular segments of reinforced concrete that were cast in place. Thick ribs provide additional support. Modern concrete shell constructionModern thin concrete shells, which began to appear in the 1920s, are made from thin steel reinforced concrete, and in many cases lack any ribs or additional reinforcing structures, relying wholly on the shell structure itself.Shells may be cast in place, or pre-cast off site and moved into place and assembled. 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 (resembling concrete Quonset huts) are also possible using similar construction methods.

Geodesic domes may be constructed from concrete sections, or may be constructed of a lightweight foam with a layer of concrete applied over the top. The advantage of this method is that each section of the dome is small and easily handled. The layer of concrete applied to the outside bonds the dome into a semi-monolithic structure.Monolithic domes are cast in one piece out of reinforced concrete, and date back to the 1960s. Advocates of these domes consider them to be cost-effective and durable structures, especially suitable for areas prone to natural disasters. They also point out the ease of maintenance of these buildings. Monolithic domes can be built as homes, office buildings, or for other purposes.

BARREL VAULTA barrel vault, also known as a tunnel vault or a wagon vault, is an architectural element formed by the extrusion of a single curve (or pair of curves, in the case of a pointed barrel vault) along a given distance. The curves are typically circular in shape, lending a semi-cylindrical appearance to the total design. The barrel vault is the simplest form of a vault: effectively a series of arches placed side by side, i.e., one after another.As with all arch-based constructions, there is an outward thrust generated against the walls underneath a barrel vault. There are several mechanisms for absorbing this thrust. One is, of course, to make the walls exceedingly thick and strong - this is a primitive and sometimes unacceptable method. A more elegant method is to build two or more vaults parallel to each other; the forces of their outward thrusts will thus negate each other. This method was most often used in construction of churches, where several vaulted naves ran parallel down the length of the buildingBarrel vaults are known from Ancient Egypt, and were used extensively in Roman architecture. They were also used to replace the Cloaca Maxima with a system of underground sewers. Early barrel vault designs occur in northern Europe, Turkey, Morocco and other regions. In medieval Europe the barrel vault was an important element of stone construction in monasteries, castles, tower houses and other structures. This form of design is observed in cellars, crypts, long hallways, cloisters and even great halls.

Barrel vaulting was known and employed by early civilizations, including Ancient Egypt and Mesopotamia, but apparently it was not a very popular or very common method of construction. The technique probably evolved out of necessity to roof buildings with masonry elements such as bricks or stone blocks in areas where timber and wood were scarce. The earliest known example of a vault is a Tunnel vault found under the Sumerian ziggurat at Nippur in Babylonia, ascribed to about 4000 BC, which was built of burnt bricks amalgamated with clay mortar. The earliest tunnel vaults in Egypt are found at Requagnah and Denderah, circa 3500 BC; these were built in sun-dried brick in three rings over passages descending to tombs: in these cases, as the span of the vault was only two meters.

Ancient Romans most probably inherited their knowledge of barrel vaulting from Etruscans. Romans were the first to use this building method extensively on large-scale projects, and were probably the first to use scaffolding to aid them in construction of vaults spanning over widths greater than anything seen before.

After the fall of the Roman empire, few buildings large enough to require much in the way of vaulting were built for several centuries. In the early Romanesque period, a return to stone barrel vaults was seen for the first great cathedrals; their interiors were fairly dark, due to thick, heavy walls needed for support of vault. One of the largest and most famous churches enclosed from above by a vast barrel vault was the church of Cluny Abbey, built between eleventh and twelfth century.In thirteenth and fourteenth century, with the advance of the new Gothic style, barrel vaulting became almost extinct in constructions of great Gothic cathedrals; groin vaults reinforced by stone ribs were mostly used in the beginning, and later on various types of spectacular, ornate and complex medieval vaults were developed. However, with the coming of Renaissance and Baroque, and revived interest in art and architecture of antiquity, barrel vaulting was re-introduced on a truly grandiose scale, and employed in constructions of many famous buildings and churches, such as Basilica di Sant'Andrea di Mantova by Leone Battista Alberti, San Giorgio Maggiore by Andrea Palladio, and perhaps most glorious of all, St. Peter's Basilica in Rome, where a huge barrel vault spans the 27 meters wide nave.

ENGINEERING ISSUES

With a barrel vault design the vectors of pressure result in a downward force on the crown while the lower portions of the arches realise a lateral force pushing outwards. As an outcome this form of design is subject to failure unless the sides are anchored or buttressed to very heavy building elements or substantial earthwork sidings. For example, at Muchalls Castle in Scotland adjacent walls to the barrel vaulted chambers are up to 4,6 meters thick, adding the buttressing strength needed to secure the curved design.Well documented cases exist of the long term stress effects on inadequately laterally supported barrel vaults such as the seventeenth century church of Guimarei.[5]