Masonry Magazine October 1995 Page. 33
Structural Masonry Arches
Figure 1 depicts examples of structural masonry arches used in contemporary construction. An arch is normally classified by the curve of its intrados and by its function, shape or architectural style. Figure 2 illustrates some of the many different brick masonry arch types. Jack, segmental, semicircular and multicentered arches are the most common types used for building arches. For very long spans and for bridges, semicircular arches are often used because of their structural efficiency.
Mainly due to their variety of components and elements, arches have developed their own set of terminology. Following is a glossary of arch terminology. Figure 3 illustrates many of the terms defined in this glossary. Technical Notes in this series will use this terminology.
Arch Terminology
Abutment: The masonry or combination of masonry and other structural members which support one end of the arch at the skewback.
Arch: A form of construction in which masonry units span an opening by transferring vertical loads laterally to adjacent voussoirs and, thus, to the abutments. Some common arch types are as follows:
Blind: An arch whose opening is filled with masonry.
Bullseye: An arch whose intrados is a full circle. Also known as a Circular arch.
Elliptical: An arch with two centers and continually changing radii.
Fixed: An arch whose skewback is fixed in position and inclination. Masonry arches are fixed arches by nature of their construction.
Gauged: An arch formed with tapered voussoirs and thin mortar joints.
Gothic: An arch with relatively large rise-to-span ratio, whose sides consist of arcs of circles, the centers of which are at the level of the spring line. Also referred to as a Drop, Equilateral or Lancet arch, depending upon whether the spacings of the centers are respectively less than, equal to or more than the clear span.
Horseshoe: An arch whose intrados is greater than a semicircle and less than a full circle. Also known as an Arabic or Moorish arch.
Jack: A flat arch with zero or little rise.
Multicentered: An arch whose curve consists of several arcs of circles which are normally tangent at their intersections.
Relieving: An arch built over a lintel, jack arch or smaller arch to divert loads, thus relieving the lower arch or lintel from excessive loading. Also known as a Discharging or Safety arch.
Segmental: An arch whose intrados is circular but less than a semicircle.
Semicircular: An arch whose intrados is a semicircle (half circle).
Slanted: A flat arch which is constructed with a keystone whose sides are sloped at the same angle as the skewback and uniform width brick and mortar joints.
Triangular: An arch formed by two straight, inclined sides.
Tudor: A pointed, four-centered arch of medium rise-to-span ratio whose four centers are all beneath the extrados of the arch.
Venetian: An arch formed by a combination of jack arch at the ends and semicircular arch at the middle. Also known as a Queen Anne arch.
Camber: The relatively small rise of a jack arch.
Centering: Temporary shoring used to support an arch until the arch becomes self-supporting.
Crown: The apex of the arch's extrados. In symmetrical arches, the crown is at the midspan.
Depth: The dimension of the arch at the skewback which is perpendicular to the arch axis, except that the depth of a jack arch is taken to be the vertical dimension of the arch at the springing.
Extrados: The curve which bounds the upper edge of the arch.
Intrados: The curve which bounds the lower edge of the arch. The distinction between soffit and intrados is that the intrados is a line, while the soffit is a surface.
Keystone: The voussoir located at the crown of the arch. Also called the key.
Label Course: A ring of projecting brickwork that forms the extrados of the arch.
Rise: The maximum height of the arch soffit above the level of its spring line.
Skewback: The surface on which the arch joins the supporting abutment.
Skewback Angle: The angle made by the skewback from horizontal.
Soffit: The surface of an arch or vault at the intrados.
Span: The horizontal clear dimension between abutments.
Spandrel: The masonry contained between a horizontal line drawn through the crown and a vertical line drawn through the upper most point of the skewback.
Springing: The point where the skewback intersects the intrados.
Springer: The first voussoir from a skewback.
Spring Line: A horizontal line which intersects the springing.
Voussoir: One masonry unit of an arch.
Arch Terms
EXTRADOS
DEPTH
CROWN KEYSTONE
RISE
SKEWBACK
ANGLE
SPRING LINE
INTRADOS
SPRINGING
VOUSSOIR
SPAN
SKEWBACK
ABUTMENT
Arch Terms
FIG. 3
MASONRY-SEPTEMBER/OCTOBER, 1995 33