Arch structures

An arch is a curved structure capable of spanning a space while supporting significant weight (e.g. a doorway in a stone wall). The arch was developed in Mesopotamia, Assyria Egypt and Etruria. It was later refined in Ancient Rome. The arch became an important technique in cathedral building and is still used today in some modern structures as for example in bridges.

History

Before the development of the arch, the principal method of spanning a space was the simple post-and-beam construction , in which a horizontal beam is supported by two columns. This type of construction was used to build the great Greek temples. The columns of these temples are closely spaced because of the limited length of available stones. Much larger spans can now be achieved using steel beams, but the spans are limited because the beams tend to sag under heavy loads.Arches were used by the Egyptian, Babylonian, Greek and Assyrian civilizations for underground structures such as drains and vaults, but the ancient Romans were the first to use them widely above ground. The so-called Roman arch is semicircular, and built from an odd number of arch bricks (in modern architectural parlance, these are called voussoirs). The capstone or keystone is the topmost stone in the arch. This shape is the simplest to build, but not the strongest. There is a tendency for the sides to bulge outwards, which must be counteracted by an added weight of masonry to push them inwards.

The semicircular arch can be flattened to make an elliptical arch. The horseshoe arch is based on the semicircular arch, but its lower ends are extended further round the circle until they start to converge. It was used in Spanish Visigothic architecture, Islamic architecture, as in the Great Mosque of Damascus and in later Moorish buildings. It was used for decoration rather than for strength. The semicircular arch was followed in Europe by the pointed Gothic arch or ogive, whose centreline more closely followed the forces of compression and which was therefore stronger. This design had been used by the Assyrians as early as 722 BC. The parabolic and catenary arches are now known to be the theoretically strongest forms.Another important architectural innovation was the pointed Gothic arch. This type of structure was first used in Europe beginning in the 12th century, followed by the construction of several magnificent Gothic cathedrals in France in the 13th century. One of the most striking features of these cathedrals is their extreme height.

Construction

An arch requires all of its elements to hold it together. This raises the interesting question of how an arch is actually constructed. One simple answer is to build a frame (historically, of wood) which exactly follows the form of the underside of the arch. This is known as a centre or centring. The voussoirs are laid on it until the arch is complete and self-supporting. For an arch higher than head height, scaffolding would in any case be required by the builders, so the scaffolding can be combined with the arch support.

Arches Bridges

After girders, arches are the second oldest bridge type and a classic structure. Unlike simple girder bridges, arches are well suited to the use of stone. Many ancient and well know examples of stone arches still stand to this day. Arches are good choices for crossing valleys and rivers since the arch doesn't require piers in the center. Arches can be one of the more beautiful bridge types.

Arch nowadays

With the advent of more advanced methods of structural analysis, it has become possible to determine the optimum shape of an arch under given load conditions

Framed Structures

A framed structure in any material is one that is made stable by a skeleton that is able to stand by itself as a rigid structure without depending on floors or walls to resist deformation.Alaska Building, Seattle's first steel-framed skyscraper, is completed in 1904.From 1903 to 1904, Seattle's tallest building and first steel-framed skyscraper is constructed. The Alaska Building, located at the southeast corner of 2nd Avenue and Cherry Street, rises 14 stories high.Earmes & Young, a St. Louis architectural firm, with assistance by Seattle architects Saunders & Lawton, designed the building. The Alaska Building remained the tallest building in Seattle until the 18 story Hoge Building (705 2nd Avenue) was completed in 1911.Materials such as wood, steel, and reinforced concrete, which are strong in both tension and compression, make the best members for framing. Masonry skeletons, which cannot be made rigid without walls, are not frames. The heavy timber frame, in which large posts, spaced relatively far apart, support thick floor and roof beams, was the commonest type of construction in eastern Asia and northern Europe from prehistoric times to the mid-19th century. It was supplanted by the American light wood frame (balloon frame), composed of many small and closely spaced members that could be handled easily and assembled quickly by nailing instead of by the slow joinery and dowelling of the past. Construction is similar in the two systems, since they are both based on the post-and-lintel principle. Posts must rest on a level, waterproof foundation, usually composed of masonry or concrete, on which the sill (base member) is attached. Each upper story is laid on crossbeams that are supported on the exterior wall by horizontal members. Interior walls give additional beam support.In the heavy-timber system, the beams are strong enough to allow the upper story and roof to project beyond the plane of the ground-floor posts, increasing the space and weather protection. The members are usually exposed on the exterior. In China, Korea, and Japan, spaces between are enclosed by light screen walls and in northern Europe partly by thinner bracing members and partly by boards, panels, or (in half-timbered construction) bricks or earth.The light frame, however, is sheathed with vertical or horizontal boarding or shingling, which is jointed or overlapped for weather protection. Sheathing helps to brace as well as to protect the frame, so the frame is not structurally independent as in steel frame construction. The light-frame system has not been significantly improved since its introduction, and it lags behind other modern techniques. Prefabricated panels designed to reduce the growing cost of construction have not been widely adopted. Modern heavy-timber and laminated-wood techniques, however, provide means of building up compound members for trusses and arches that challenge steel construction for certain large-scale projects in areas where wood is plentiful.Steel framing is based on the same principles but is much simplified by the far greater strength of the material, which provides more rigidity with fewer members. The load-bearing capacity of steel is adequate for buildings many times higher than those made of other materials. Because the column and beam are fused by riveting or welding, stresses are distributed between them, and both can be longer and lighter than in structures in which they work independently as post-and-lintel. Thus, large cubic spaces can be spanned by four columns and four beams, and buildings of almost any size can be produced by joining cubes in height and width. Since structural steel must be protected from corrosion, the skeleton is either covered by curtain walls or surfaced in concrete or, more rarely, painted. The steel frame is used also in single-story buildings where large spans are required. The simple cube then can be abandoned for covering systems employing arches, trusses, and other elements in a limitless variety of forms in order to suit the functions of the building.