Arch
|
Isometric view of a typical arch |
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 first developed in the
Indus Valley civilization circa
2500 BC [ http://www.krysstal.com/inventions_04.html] and subsequently in
Mesopotamia,
Egypt,
Assyria,
Etruria, and later refined in
Ancient Rome. The arch became an important technique in
cathedral building and is still used today in some modern structures such as
bridges.
Arches were used by the Harappan,
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 although it is thought that Romans learned it from the
Etruscans. The arch was used in some bridges in
China since the
Song dynasty.
The so-called Roman arch is semicircular, and built from an odd number of arch bricks (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 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.
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 and
mudéjar architecture, as in the Great
Mosque of
Damascus and in later
Moorish buildings. It was used for decoration rather than for strength.
The arch was especially important to the construction of the great systems of aqueducts by the ancient Romans. These consisted of long series of arches, a much more economical mode of construction than, say, a continuous wall of support.
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. Occasionally arches would fall down when the frame was removed if construction or planning had been incorrect. (The
A85 bridge at Dalmally, Scotland suffered this fate on its first attempt, in the 1940s).
The following gallery shows examples of arch forms displayed in roughly the order in which they were developed.
The arch is significant because, in theory at least, it provides a structure which eliminates
tensile stresses in spanning an open space. All the forces are resolved into
compressive stresses. This is useful because several of the available building materials such as
stone,
cast iron and
concrete can strongly resist compression but are very weak when
tension,
shear or
torsional stress is applied to them. By using the arch configuration, significant spans can be achieved. The arch is a very useful structure as it is completely self-supporting. This is because all the compressive forces hold it together in a state of equilibrium. This even applies to frictionless surfaces.
This same principle holds when the
force acting on the arch is not vertical such as in spanning a doorway, but horizontal, such as in arched
retaining walls or
dams.
Even when using concrete, where the structure may be monolithic, the principle of the arch is used so as to benefit from the concrete's strength in resisting compressive stress. Where any other form of stress is raised, it has to be resisted by carefully placed
reinforcement rods or fibres. (See
Arch bridge.)
A
vault is an application of the arch extended horizontally in two dimensions; the
groin vault is the intersection of two vaults.
A
dome is a three-dimensional application of the arch, rotated about the center axis.
Igloos are notable early structures making use of domes.
A special form of the arch is the
triumphal arch, usually built to celebrate a victory in war. The most famous example of this is the
Arc de Triomphe in
Paris,
France.
Natural rock formations may also be referred to as "arches". These
natural arches are formed by
erosion rather than being carved or constructed by man. See
Arches National Park for examples.
In the
Star Trek universe, you say "arch" to get the arch on the
holodeck to appear.
* pp. 27-8
