France: Roman aqueductPont du Gard, an ancient Roman aqueduct in Nîmes, France.© Karel Gallas/Shutterstock.com
Vault
The evolution of the vault begins with the discovery of the arch, because the basic “barrel” form, which appeared first in ancient Egypt and the Near East, is simply a deep, or three-dimensional, arch. Since the barrel vault exerts thrust as the arch does, it must be buttressed along its entire length by heavy walls in which openings must be limited in size and number. This is a disadvantage, since it inhibits light and circulation.
barrel vaultBarrel vault in Aranjuez, Spain.© Antonio Ovejero Diaz/Shutterstock.com
But Roman builders discovered that openings could be made by building two barrel vaults that intersected at right angles to form the groin vault, which is square in plan and may be repeated in series to span rectangular areas of unlimited length. This vault has the additional advantage that its thrusts are concentrated at the four corners, so that the supporting walls need not be uniformly massive but may be buttressed where they support the vault.
Two disadvantages of the groin vault encouraged Gothic builders to develop a modification known as the rib vault. First, to build a groin vault, a form must be made to pour or lay the entire vault, and this requires complex scaffolding from the ground up; second, the groin vault must be more or less square, and a single vault cannot span extended rectangular areas. The rib vault provided a skeleton of arches or ribs along the sides of the area and crossing it diagonally; on these the masonry of the vault could be laid; a simple centring sufficed for the ribs. To cover the rectangular areas, the medieval mason used pointed arches, which, unlike round arches, can be raised as high over a short span as over a long one. Thus, the vault could be composed of the intersection of two vaults of different widths but the same height.
To reduce further the thickness of the wall (to the point of substituting large areas of glass for masonry), Gothic builders developed the flying buttress, which counteracts vault thrust not by continuous wall mass and weight but by counterthrust created by exterior half-arches placed at the height of the vaults at the points of greatest stress. These buttresses conduct stresses to heavier wall buttresses below the window level.
flying buttressTwo flying buttresses on the abbey of Bath, England.Adrian Pingstone
The next important development in vaults, as in arches, came with 19th-century materials. Great iron skeleton vaults were constructed as a framework for light materials such as glass (Crystal Palace, London). The elimination of weight and excessive thrust, the freedom in the use of materials, and the absence of centring problems favoured the simple barrel vault and made more complex types obsolete. But in many of the modern frame systems the vault itself loses its structural function and becomes a thin skin laid over a series of arches.
The transept of the Crystal Palace, designed by Sir Joseph Paxton, at the Great Exhibition of 1851, Hyde Park, London.Hulton Archive/Getty Images
While the arch is supplanting the vault in one area of technique, the vault has abandoned the arch principle in another. The reinforced-concrete shell vault, based on the principle of the bent or molded slab, is one of the most important innovations in the history of architecture. It has all the advantages of load distribution of the concrete floor slab, plus the resistance to bending provided by its curved form. The shell is reinforced in such a way that it exerts no lateral thrust and may be supported as if it were a beam or truss; hence, the form no longer necessitates the conducting of loads into the wall, and the vault may be designed with great freedom. Dome
Domes appeared first on round huts and tombs in the ancient Near East, India, and the Mediterranean region but only as solid mounds or in techniques adaptable only to the smallest buildings. They became technically significant with the introduction of the large-scale masonry hemispheres by the Romans. Domes, like vaults, evolved from the arch, for in their simplest form they may be thought of as a continuous series of arches, with the same centre. Therefore, the dome exerts thrusts all around its perimeter, and the earliest monumental examples required heavy walls. Since the walls permitted few openings and had to be round or polygonal to give continuous support, early domes were difficult to incorporate into complex structures, especially when adjacent spaces were vaulted.
Byzantine architects perfected a way of raising domes on piers instead of walls (like groin vaults), which permitted lighting and communication from four directions. The transition from a cubic plan to the hemisphere was achieved by four inverted spherical triangles called pendentives—masses of masonry curved both horizontally and vertically. Their apexes rested on the four piers, to which they conducted the forces of the dome; their sides joined to form arches over openings in four faces of the cube; and their bases met in a complete circle to form the dome foundation. The pendentive dome could rest directly on this foundation or upon a cylindrical wall, called a drum, inserted between the two to increase height.
The dome was unsuited to the lightness and verticality of late medieval styles but was widely used in the Renaissance and Baroque periods. Renaissance builders adapted the Gothic rib system to dome construction and found new means to reduce loads and thrust (concentric chains, etc.) that permitted high drums and variations in the curvature of the dome. The awkward, tunnellike effect produced on the interior by high domes was often hidden by an internal shell built on the same foundations (as at Florence Cathedral and St. Paul’s Cathedral, London).
Florence: cathedralCathedral of Santa Maria del Fiore, Florence.© Ron Gatepain (A Britannica Publishing Partner)
The effort and ingenuity devoted to doming rectangular buildings can be explained principally by the symbolic character of the form, since vaulting is a simpler alternative. So it was chiefly the desire to observe tradition that preserved the dome in the early era of iron and steel construction, and, with rare exceptions (Halle aux Blés, Paris; the Coal Exchange, London), 19th-century examples retained masonry forms without exploiting the advantages of metal.
Newer techniques, however, have added practically to the expressive advantages of domes. The reinforced-concrete slab used in vaulting can be curved in length as well as width (like an inflated handkerchief or a parachute). And in this development the distinction between vaults and domes loses significance, being based on nothing but the type of curvature in the slab. Geodesic domes, developed in the 20th century by Fuller" class="md-crosslink">R. Buckminster Fuller, are spherical forms in which triangular or polygonal facets composed of light skeletal struts or flat planes replace the arch principle and distribute stresses within the structure itself, as in a truss. Geodesic domes can be supported by light walls and are the only large domes that can be set directly on the ground as complete structures.