triumphal arch, monumental structure embodying one or more arched passages, frequently built to span a road and designed to honor a king or general or to commemorate a military triumph. This form of monument was probably invented by the Romans, who built them throughout the empire. Examples exist in Italy, France, Spain, Asia Minor, and North Africa, dating from the empire. The typical Roman triumphal arch had a single arched opening in the earliest examples, e.g., the Arch of Titus, Rome (A.D. 81); after the 2d cent. a large arch flanked by two smaller ones became common. The piers were faced with columns and enriched with sculptures or bas-reliefs relating to the events commemorated, while above the entablature was an attic story for dedicatory inscriptions supporting a quadriga, a sculptured four-horse chariot group. Among the Roman arches remaining are that of Trajan, at Benevento, Italy (114), relating the story of the emperor's life, and those of Septimius Severus (203) and of Constantine (c.315) at Rome, honoring the military victories of the two emperors. In modern times some arches have been built to celebrate military triumphs. Among them in Paris are the Porte Saint-Denis and the Porte Saint-Martin, both erected under the reign of Louis XIV, and the Arc de Triomphe du Carrousel and the Arc de Triomphe de l'Étoile, both built at the decree of Napoleon I. Other well-known arches are the Brandenburg Gate in Berlin; the Victory Gate in Munich; the Marble Arch in London; and the Washington Arch in New York City.

arch, the spanning of a wall opening by means of separate units (such as bricks or stone blocks) assembled into an upward curve that maintains its shape and stability through the mutual pressure of a load and the separate pieces. The weight of the supported load is thus converted into downward and outward lateral pressures called thrusts, which are received by the solid piers (abutments) flanking the opening. The blocks, called voussoirs, composing the arch usually have a wedge shape but they can be rectangular with wedge-shaped joints between them. The underside of the arch is the intrados or soffit and the upper surface above the crown block (keystone) of the arch is the extrados. The point where the arch starts to curve is the foot of the arch, and the stones there are the springers. The surface above the haunch (just below the beginning of the curve) contained within a line drawn perpendicular to the springing line (from which the arch curves), and another drawn horizontal to the crown is the spandril. In modern fireproof construction the word arch is also used for the masonry that fills the space between steel beams and acts as a floor support. The arch was used by the Egyptians, Babylonians, and Greeks, chiefly for underground drains, and also by the Assyrians in the construction of vaulted and domed chambers. In Europe the oldest known arch is the Cloaca Maxima, the huge drain at Rome built by Lucius Tarquinius Priscus c.578 B.C. The Romans developed the semicircular arch, modeled on earlier Etruscan structures, in the vaults and domes of their monumental buildings. Its use was continued in early Christian, Byzantine, and Romanesque architecture. In the 13th cent. the pointed arch (used as early as 722 B.C. in Assyrian drains) came into general use. The contact of Europeans with Saracenic architecture during the Crusades is offered among other theories for its introduction into Europe. But it is likely that the pointed arch may have been independently rediscovered in Europe in the Middle Ages as a device for solving many of the mechanical difficulties of vault construction. Its adoption was an essential element in the evolution of the Gothic system of design. With the Renaissance there was a return to the round arch, which prevailed until the 19th-century invention of steel beams for wide spans relegated the arch to a purely decorative function. Although the circular and pointed forms have predominated in the West, the Muslim nations of the East developed a variety of other arched shapes, including the ogee arch used in Persia and India, the horseshoe arch used in Spain and North Africa, and the multifoil or scalloped arch used especially in the Muslim architecture of Spain. In the 20th cent. arches often take a parabolic shape. They are usually constructed with laminated wood or reinforced concrete, materials that give greater lightness and strength to the structure. See triumphal arch.

Titus, Arch of: see Titus, Roman emperor.

Arch, Joseph, 1826-1919, English labor leader, a Primitive Methodist preacher. He founded the National Agricultural Labourers Union in 1872 and became its president. In 1873, Arch visited Canada and the United States to study labor and immigration problems. He served (1885-86, 1892-1900) as one of the first labor members in Parliament and was instrumental in enfranchising agricultural workers.

Monumental structure, originating in Rome, pierced by at least one arched passageway and erected to honour an important person or commemorate a significant event. It usually spanned a street or roadway and was built astride the line of march of a victorious army during its triumphal procession. Most were built during the empire period. The basic form consisted of two piers connected by an arch and crowned by a superstructure, or attic, that served as a base for statues and bore inscriptions. The large central arch could also be flanked by two smaller arches. The Roman triumphal arch had a facade of marble columns, and the archway and sides were adorned with relief sculpture. Among those built since the Renaissance is the Arc de Triomphe in Paris.

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or natural arch

Naturally created arch formation resembling a bridge. Most are erosion features that occur in sandstone or limestone. Some are formed by the collapse of part of a cavern roof. Others may be produced by entrenched rivers eroding through meander necks to form cutoffs. Still others are produced by exfoliation (separation of successive thin shells) and may be enlarged by wind erosion.

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Curved structure that spans the opening between two piers or columns and supports loads from above. The masonry arch provides the stepping stone from the post-and-beam system to the evolution of the vault, and was first widely used by the Romans. Its construction depends on a series of wedge-shaped blocks (voussoirs) set side by side in a semicircular curve or along two intersecting arcs (as in a pointed arch). The central voussoir is called the keystone, and the two points where the arch rests on its supports are known as the spring points. An arch can carry a much greater load than a horizontal beam of the same size and material, because downward pressure forces the voussoirs together instead of apart. The resulting outward thrust must be resisted by the arch's supports. Present-day lightweight monolithic (one-piece) arches of steel, concrete, or laminated wood are highly rigid, and thereby minimize horizontal thrust.

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