Glossary of Stone Arch Bridge Terms

This is a glossary of common stone arch bridge terms. In some instances the definitions are further clarified by labeled photographs located at the bottom of the glossary. Further information is frequently provided by links that go either to a post specifically about the topic or one that has relevant information.

• Abutment: The abutment is the support on which the arch rests. Frequently it consists of a solid platform of stone, but it is important to point out that some bridges with arches sprung at the streambed level on a rock-bottomed stream may, in fact, use the rock of the streambed itself as an abutment. Another variant is to build the abutment in the form of a box into which fill is placed and upon which the arch rests. For low-rise arches the abutment must be heavy enough to resist the horizontal thrust of the arch in order to prevent the arch from sliding out at the ends and collapsing.
• Approach: The approach is the section of a stone arch bridge that carries the road to the bridge proper. The approach is held up by two parallel walls, which are usually directly bonded to the spandrel walls, and are a direct continuation of the bridge. The approaches were usually built to carry the road over periodically flooding areas and/or as a ramp to carry the road up to the level of the arch.
• Arch: The arch is the arrangement of stone which transfers its own weight and any weight above it down to the abutments upon which it rests. The arch’s strength is obtained by the curved line of stones, which holds the structure together by compression.
• Arch Faces: The outside faces of the arch; the faces are the side surfaces of the arch located on the upstream and downstream sides of a bridge. The faces of a stone bridge are often well shaped as they are vulnerable to debris impacts.
• Arch Ring: The arch ring is the stones that collectively which form the arch.
• Backing: Backing broadly refers to the fill placed above the abutments and behind the arch. Backing often refers more to solid material as opposed to loose dirt, commonly being made of loose stone masonry. Such backing, called solid backing, significantly strengthens a stone arch bridge, contributing to the solidity of the structure.
• Basket-handle Arch: A broad category of arches so called because of their resemblance in curvature to the handle of a basket. Though the exact curve varies considerably, broadly speaking basket-handle arches tend to be heavily rounded at the ends, then flatten out into a long, low curve towards the middle of the span.
• Centering: The temporary formwork used to support an unfinished arch during construction.
• Course: Refers to a level of stonework reached by the parallel placement of relatively evenly thick stones.
• Cramp: A metal bar, usually shaped like an oversized staple, used to bind two stones together.
• Crown: The crown is the topmost portion of the arch; it is centered around and includes the keystone.
• Curbing: Refers to the top line of stone or, sometimes, cement located atop a stone bridge. The curbing rises above the fill level, marks the ends of the bridge, and helps to prevent vehicles from driving off. Often, the curbing was mounded with some form of railing; gas pipe was often mounted into the curbing of a stone bridge in the early 1900s to form a rail along the top of the bridge.
• Cutwater: The cutwater is a carefully shaped protuberance placed on the upstream and/or downstream side of a pier to aid in the smooth flow of water beneath the bridge. The cutwaters, commonly triangular on a stone bridge, also help prevent the accumulation of debris around a pier.
• Elliptical Arch: An basket-handle arch with a curve drawn from an ellipse.
• Extrados: Refers to the upper surface of an arch. Looking at the crown of the arch, the extrados would be the top surface of the arch ring.
• Fill: The material that is placed over the arches, piers, abutments, etc. of a stone arch bridge in order to create a usable road surface. The fill also helps to distribute the weight of vehicles crossing a stone bridge.
• Gothic Arch: An arch that consists of a set of separate arcs, typically two, that join at the crown of the arch to form a point. This type of arch was quite popular in medieval times, most notably for cathedrals but also for stone arch bridges.
• Grout: Liquid mortar solution usually poured or injected into the joints between stones.
• Haunch: The portion of the arch located above the springing but below the crown; often refers to the area around the 1/4 span sections of the arch.
• Intrados: Refers to the underside surface of an arch. Looking at the crown of the arch, the intrados would be the bottom surface of the arch ring.
• Joints: The cavities and seams between individual stones in a masonry structure.
• Keystone: The keystone is the topmost stone of an arch located in the center of the crown. It is almost invariably the last stone to be placed in an arch, and frequently features special treatment, such as large size.
• Masonry: Refers to the stones and their collective placement. There are several forms of masonry, which vary primarily by the amount of work done to the stones to achieve a good fit. The different types are rubble, squared-stone, and ashlar.
• Mortar: A semiliquid compound similar to concrete. Mortar is placed between stones to fill in all the cavities between the stones. As it drys, the mortar adheres to the stones and hardens into a solid, thereby turning the masonry into a cohesive mass.
• Pier: A support, usually placed in the stream, which supports two arches in a multi-span stone arch bridge. Though some piers are large enough to totally support an arch (for example, abutment piers), they often are so relatively thin that they rely on the combined weight and thrust of both arches on the pier to make the structure stable.
• Pile: A deeply driven column placed beneath an abutment or pier to help stabilize it and prevent settling. Traditionally, the piles placed beneath a stone bridge were made of wood, and relied on being completely and perpetually submerged and buried to prevent deterioration.
• Pointing: The insertion of mortar between the outside joints in masonry both as a finish and as a repair for damaged mortar.
• Ribs: A series of relatively thin arches placed parallel to each other across which flagstones are laid to create a full-width arch.
• Rise: The height between the top point of the crown of an arch and its springing.
• Roman Arch: An arch with a curve equivalent to one half of a circle.
• Scour: Erosion of the streambed caused by flowing water. Scour usually refers specifically to this erosion as occurs to the ground below bridge foundations, thereby threatening the entire structure.
• Scour Apron: A barrier, usually of concrete, placed around the piers and/or abutments of a bridge in order to hinder erosion around the foundations. Sometimes such an apron around a pier may be referred to as a “pontoon.”
• Segmental Arch: An arch with a curve equivalent to some value less than half of a circle.
• Sheeting: Though sheeting specifically refers to the stones laid across the ribs of a ribbed arch, it can also refer to the portion of an arch between the two arch faces.
• Skewback: An angled stone upon which a segmental arch springs.
• Soffit: The underside of an arch.
• Span: The total distance from the two ends of an arch; how long of a distance an arch covers.
• Spandrel: The area of a stone bridge above an arch between the center of the crown and the springing. This area is usually covered with fill held in by two walls known as spandrel walls in order to create a usable roadbed.
• Springing: The point of a bridge at which the curve of the arch bridge begins. Also refers to the horizontal plane on which the arches of the bridge rest.
• Tie-rod: A metal bar passed through a masonry structure to help prevent loose sections from separating. The tie-rod resembles a very long bolt, and is equipped with large metal plates to help it contact a larger surface area. The tie-rod is most commonly seen in stone arch bridges passed through both spandrel walls, thereby tying them together in order to prevent them from pushing out.
• Thrust: The force exerted by the weight of an arch, which can be either vertical, horizontal or, more commonly, some combination thereof.
• Wing Wall: Refers to a set of walls joining the bridge and laid along the streambanks a distance in order to minimize streambank erosion at the bridge. The term wing walls also frequently refers to the walls that hold in the approach of a stone bridge.