Jack arch
Encyclopedia
A jack arch is a structural element in masonry
construction that provides support at openings in the masonry. Alternate names are "flat arch" and "straight arch".
Unlike regular arches, jack arches are not semicircular in form. Instead, they are flat in profile and are used under the same circumstances as lintels. Unlike lintels, which are subject to bending stress, jack arches are composed of individual masonry elements cut or formed into a wedge shape that efficiently uses the compressive strength of the masonry in the same manner as a regular arch
. Like regular arches, jack arches require a mass of masonry to either side to absorb the considerable lateral thrust created by the jack arch.
Jack arches have the advantage of being constructed from relatively small pieces of material that can be handled by individuals, as opposed to lintels which must necessarily be monolithic and which must be oversized unless reinforced by other means.
In small-scale brick masonry projects, jack arches are typically sawn from an appropriately-sized fired-clay lintel, giving a more precise and consistent joint width than field-sawn shapes.
There is considerable scope for incorporation of decorative patterns and elements into jack arches. keystone
s, stepped or arched top profiles and polychrome
or contrasting colors and materials may all be used to create the desired effect.
Masonry
Masonry is the building of structures from individual units laid in and bound together by mortar; the term masonry can also refer to the units themselves. The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone; concrete block, glass block, stucco, and...
construction that provides support at openings in the masonry. Alternate names are "flat arch" and "straight arch".
Unlike regular arches, jack arches are not semicircular in form. Instead, they are flat in profile and are used under the same circumstances as lintels. Unlike lintels, which are subject to bending stress, jack arches are composed of individual masonry elements cut or formed into a wedge shape that efficiently uses the compressive strength of the masonry in the same manner as a regular arch
Arch
An arch is a structure that spans a space and supports a load. Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.-Technical aspects:The...
. Like regular arches, jack arches require a mass of masonry to either side to absorb the considerable lateral thrust created by the jack arch.
Jack arches have the advantage of being constructed from relatively small pieces of material that can be handled by individuals, as opposed to lintels which must necessarily be monolithic and which must be oversized unless reinforced by other means.
In small-scale brick masonry projects, jack arches are typically sawn from an appropriately-sized fired-clay lintel, giving a more precise and consistent joint width than field-sawn shapes.
There is considerable scope for incorporation of decorative patterns and elements into jack arches. keystone
Keystone (architecture)
A keystone is the wedge-shaped stone piece at the apex of a masonry vault or arch, which is the final piece placed during construction and locks all the stones into position, allowing the arch to bear weight. This makes a keystone very important structurally...
s, stepped or arched top profiles and polychrome
Polychrome
Polychrome is one of the terms used to describe the use of multiple colors in one entity. It has also been defined as "The practice of decorating architectural elements, sculpture, etc., in a variety of colors." Polychromatic light is composed of a number of different wavelengths...
or contrasting colors and materials may all be used to create the desired effect.