Scotland’s Oldest Bridges.
A map-based catalogue of the oldest masonry bridges in Scotland.
Parts of a Bridge and Glossary.
Glossary of components of ancient masonry bridges.
Abutment. Solid masonry structure built to support the lateral pressure at both ends of a single arch. A multi-arched bridge may have abutments at both ends, only. (Figs 1,2,3)
Archivolt.Ornamental moulding or banding surrounding an arched opening.
Arch-back. The top surface of the barrel arch, as opposed to the soffit- which is the bottom surface. On top of the arch-back, a layer of puddled clay would be pasted, for waterproofing. Above that would be infill, and finally the cobbled decking. The outer edge of the arch-back, on each side, is the extrados, where the arch-back meets the external face of the facing voussoirs.The outer edge of the soffit is the intrados.
Arris. The sharp edge of the external join between two adjoining surfaces. The most important arris on a bridge is the interface of the facing with the soffit of the arch.
Ashlar.Finely dressed, perfectly cut, precisely shaped masonry stones; usually cuboid; tapered for voussoirs. The term is also applied to general masonry built of such stone. This type of masonry permits very thin joints between the stones, fitting perfectly. Minimal treatment would be 'sawn four sides', and then the facing surface may be finely dressed, or may have a treatment of rough quarry facing, carved, polished or rendered exterior; this is for decorative effect. Usually, ashlar walls are ‘coursed' (long horizontal courses laid in parallel). Ancient weathered ashlar is less easy to identify as the joints were less precise to start with, and then became worn by the passage of time, so appearing to be wider.
Baluster. Short pillars in series, supporting a rail, thus forming a balustrade.
Batter. A battered masonry wall is built with a deliberate inward slope. On spandrels and sidewalls this stratagem might counter the slow outward pressure effect of infill and load. Telford was a great advocate of building bridges with battered walls and included this in his specifications.
Barrel. The barrel is the whole of the voussoir (ring stone) arch, usually one-voussoir thick, but sometimes two-voussoirs thick (a counter-course). The side edges(arrises) of the barrel are called the intrados and extrados, The lower surface is called the soffit and the upper surface is the arch-back.
Bond Stone. (perpend stone, tie stone, through stone) A masonry stone which may be ashlar or rubble, which lies transversely through an entire wall, to lock the layers together. Rubble walls were usually built in two layers with the space between them filled with small stones. Walls are usually 'bonded' every five or six feet.
Brander.(Grating, Creche)Foundation for piers.More common in Scotland. An artificial island created by sinking a rubble filled wooden frame on to the required spot.
Buttress.A masonry block attached to a wall, often at regular intervals, serving to strengthen it.
Centering. A type of false=works, a temporary wooden structure, used during construction of a bridge, to support the spanning of the gap and hold the voussoirs in place until the structure is complete. There is a misconception that the keystone has to be in place before the centering is ‘struck’ (removed). In fact, every voussoir in the ring must be in place.
Chamfer. Flat surface created by cutting off the facing edge of a squared stone voussoir. Common on 16th century voussoirs.
Cofferdam. Originally, a Roman contrivance to create dry space for the construction of piers, underwater(Old Scots: bulwark. Old French:bâtardeau). A gin and ram was employed to drive piles into the mud, adjacent to each other, until a full semi-watertight circle was completed, perhaps 50ft. in diameter, then the central pool was emptied by chains of men with ladders and scoops.
Coping Stone. (Capstone) Coping is flat or moulded masonry which sits on the top of a wall or parapet to provide weather protection and waterproofing.
Corbel. A weight carrying masonry joist, protruding from a wall and embedded within it, thus counteracting the levering effect of the supported weight; a type of masonry bracket, but often seen in continuous courses along the wall, sometimes supporting a platform or corbel table . A console is similar to a corbel but not embedded to any extent; ie. less structural and more decorative in function.
Corbel arch. Aprimitive type of arch comprising opposing sets of corbels overlapping each other, each approaching its opposite number, finally meeting at the top. This may be fashioned to create a corbel vault. A very early primitive bridging arch.
Countercourse. A second order of voussoirs.
Crown. The central highest part of an arch.
Cutwater. Pointed face of a bridge pier, splitting the water flow and thus reducing the possibility of a vortex with consequent scouring. Alberti (15th century) insisted that these should be on both the upstream and downstream sides. The Romans usually had them on upstream only.
Dentillation. An architectural decoration sometimes applied to the underside of a string course: a pattern of teeth like rectangular protrusions.
Drop Arch.A flat Gothic (pointed shape) arch in which the rise is less than half the span.
Elliptical Arch. In the shape of an ellipse, it differs from a segmental arch in that the curve springs normally from the spring line, then decreases in curvature towards the crown. A segmental arch, in contrast, has a constant curvature.
Extrados.(Fig 3)The line (on a sectional view) representing the outer, upper curve of an arch. The Intrados is the line of lower curve. The depth or thickness of the arch is the distance between.
False-works. (Centering). Temporary structure, usually of wood, used in construction to support spanning, in order to hold components in place until the structure is sufficiently advanced to support itself.
Flood Arch. An additional arch built on dry land, to cope with potential spates and so permit a flow escape during catastrophic floods. Common on ancient bridges, Flood-arches were usually smaller, but in the same style as the regular arches.
Foundations. Underpinning for the abutments and piers. Early bridge abutment foundations started with topsoil clearance, followed by either shallow or deep foundations.’ Shallow' was usually a rubble filled trench called a footing. Records of deep foundations are of iron tipped piles driven until the 'point of refusal’ (a term still used today). Roman remains of such piles were found at Corbridge; yet 1700 years later the Tay Bridge at Aberfeldy was underpinned by 1200 iron tipped oak piles. In most cases, piers were supported by foundations of Branders or Starlings.
Freestone.A type of sandstone or limestone with minimal grain, which can be cut or moulded in any direction.
Haunch.The part of an arch above the springing but below the crown.
Gothic Arch. (Ogive, Pointed). A pointed arch formed of two arc segments meeting at the crown.
Impost.(fig 3) A component of the abutment on which the first voussoir (springer) sits. In classical structures, it is the top of the column capital, which supports the entablature.
Intrados (Fig 3)The line , on a sectional view, representing the inner, lower curve of an arch. The Extrados is the line of outer curve. The depth or thickness of the arch is the distance between. The soffit is the lower exterior surface of the barrel. It is not the same as the intrados although this distinction is often blurred. The intrados is an arris (see above).
Infill.The filling material in the space above the arch barrel, between the spandrels and between the sidewalls. It completes the volume to provide a firm surface ( sometimes level) on the top decking surface of the bridge. It often consists of material excavated during the building of the foundations. Subsequent compaction induced a high strength. Waterproofing at deck level is crucial because ingress of water, followed by freezing of the infill, leads to expansion. This pushes out the spandrels. Weep holes for drainage are important.
Keystone.(Fig 1) The voussoir at the crown of the arch. There is a misunderstanding that during construction, the keystone, in particular, must be in place before temporary wooden centering is removed. In fact, every voussoir must be in place. A bridge is a barrel vault; it may be thought of as an unbroken series of arches, pressed together. Each arch ring must be intact and complete. A series of keystones runs over the crown of the barrel. The external facing keystone may be more decorative and sometimes enlarged, but in fact, it is no more a structural key than any other keystone or, for that matter, than any of the other voussoirs. The barrel must be complete before centering is removed.
Length. This is the whole length of the structure, including all arches and abutments but usually excluding approaching ramps.
Orders. Courses of voussoirs in the arch.
Parapet. A low wall on either side of the decking, contiguous with the spandrels or side walls. Parapet to parapet (‘p to p’) is useful way to quantify the width of a masonry bridge, as it describes the available road-width for traffic.
Pattress Plate.(Anchor plate, Wall Washer) An iron plate, often circular, connected to an embedded iron tie-rod, used on walls and spandrels to prevent outward leaning or bulging. Iron rails were an alternative to the plate.
Pend. (Old Scots). A pend stone was a voussoir. In later usage it referred to a vaulted entrance passage leading off the street, into a building. (Pendere: to hang.)
Pier.A vertical, load-bearing, masonry support for the adjacent ends of two arch spans; a raised structure in mid-river, sitting on foundations in the river-bed. If large enough, relative to the span of the arch, it may also serve as an abutment.
Pilaster. A decorative, rectangular, vertical wall projection, resembling a flat column protruding from a wall. Pilasters usually have capitals at the top and bases at the bottom. On bridges, they are often placed between the sidewalls and the spandrels. Pilasters are typically non-structural, as opposed to engaged columns which are supportive.
Pile. A slender column or cylinder (on ancient structures, a wooden pole - sometimes iron tipped) bored into the subsoil. Foundations were often underpinned by a series of piles, extending down to the ‘point of refusal’. This would prevent settlement and subsidence. They underpinned the foundations piers and abutments.
Pylon. An architectural decorative column, standing free, usually tapered; they are often placed at four corners of a bridge, like small obelisks. Willam Adam favoured this decoration.
Refuge. A recess or bay in the parapet providing safety to pedestrians. On 16th century bridges it was usually built above a pier and cutwater. Usually, this is a 16th century feature.
Rise.(Fig 3) Length between the centre of the springing line and the crown: this is the height of the arch itself, excluding any stilting. The rise/span ratio is a key characteristic of bridges and is a determinant of structural stability. A semicircular arch has a span equal to twice the rise (2/1). A flat segmental arch has a high span/rise ratio ( perhaps 8/1).
Rib. An arch ring projecting beneath the main arch soffit. Contstructed in advance (to economise on centering), several separated ribs would create the arch structure and then spanning stones would be laid transversely across them. Usually, this is a 16th century feature.
Rubble.Rough hewn stone, often dressed in the quarry. It is usually qualified by dressing, coursing and shape. The crudest building option is undressed uncoursed random rubble . The most refined is tooled, coursed and squared rubble . Squared (as opposed to random) rubble is crudely shaped, often with a hammer or pitching chisel. Dressings may be pitch faced (very rough), or perhaps stugged with a pointing chisel. More decorative tooled dressings are less common on bridges. Rubble mortar joints are wide, unlike those of ashlar masonry. The masonry may be completely un-coursed or laid in courses of stones of approximately the same height. An alternative is a brought-to-course arrangement which has courses about 15 inches high. each course containing haphazardly laid stones.
Scour.Erosion to foundations. An obstruction to a river such as a bridge pier, not only increases the speed of the water over a reduced space, but also causes the water-flow to dive (rather than remaining level). There may be a reversal of flow at the pier front, a vortex, which also extends around the sides of the pier in a horseshoe. This rapidly erodes the foundations of the pier, eventually leading to damage. If scour is extensive, the pier will collapse, arch spans will then shift forwards and outwards and the entire structure will be at risk. Scour also occurs, to a lesser extent, on downstream side (lea-wake) of the foundation. Well placed piles of boulders (rip-rap) and cutwaters are examples of countermeasures.
Segmental : The arch is the shape of a segment of a circle rather than a full semicircle.
Semicircular. An arch in form of a simple semicircle: sometimes referred to as a Roman arch. Typical of Roman and Romanesque Norman architecture.
Skewback. (Fig.3)angled joint between the impost and springer stone, determining the spring-angle of the arch.
Skew Arch Bridge: This type of bridge spans an obstacle at an angle in such a manner that the arch rings are not perpendicular to the abutments when viewed from above. The plan is a parallelogram.Real skew constructions have helicoidal, logaithmic or Cornes de Vache layouts requiring extensive stone cutting and shaping of the voussoirs, to fit the twisted rectangle. A false-skew does has voussoirs parallel to the abutments, but includes extra layers at each corner, viewed from above. This ‘false’ arrangement is only possible if the angle is less than 15° and even then, the prevailing pressure forces stones out at the corner. Skew bridges were not common until the railways arrived in the 1840’s; roads can easily change direction to meet a river at right angles; railracks can not usually do this. Brunstane Bridge (331467,674343) outside Edinburgh is a rare example of an 18th century skew.
Soffit.(Fig.3) The soffit is the lower exterior surface of the barrel. It is not the same as the intrados.
Span. Distance between the springers of an arch. ie. the length of the springing line.The span/rise ratio is a key characteristic of bridges and is a determinant of structural stability.
Spandrels (Figs 1,3) The parts of the side walls of a bridge, directly above the haunches of the arch, but below crown level. An open-spandrel arch will have gaps in this area, rather than walls. Spandrel walls stiffen the barrel of an arch, but only at its edges. The centre of the barrel may be more flexible then the periphery and this may lead to characteristic longitudinal cracks in old bridges.
Springer.(Fig 3) The lowest voussoir of an arch, sitting on the foundation or abutment. The springers, like keystones, run across the barrel, but at the lowest level of the soffit, on both sides.
Springing Line. A theoretical line running between the lowest edges of the facing springers; it represents the low point of the arch in section. The 'rise' of an arch is the distance between the crown and the mid point of the springing line. The springing line may at ground level, but on a stilted arch, sitting on abutments, it will be higher up.
Starling. An artificial island underpinning a pier. Common in English bridges.A cofferdam type enclosure was filled with rubble at each receding tide. In France, a concrete mix was used.
Stilted Bridge. The arch is raised on the abutments.
String Course. (Band Course) a horizontal course of masonry, usually narrower than other courses, and often protruding slightly. Extending across the structure, it is purely decorative and serves to break up the large expanses of brickwork, or perhaps delineate the break between spandrel and parapet.
Thickness. This term applies to the arch. It is is the thickness of the barrel; the distance between the intrados and the extrados; often it is the thickness of one voussoir, but only if the arch is of a single course. The ratio between the thickness and the span of a bridge is a crucial parameter of structural stability.
Voussoirs. ( Ring stones. Pend stones.) Individual wedge-shaped stones of a masonry arch. A‘pend' was an arch in old Scots.
Width. This term is applied to the width of the road surface on top of the bridge. It is usually defined as the distance between the parapet walls. ie. parapet to parapet.
Wing-wall.(Fig 1) An oblique retaining wall on the final approach to a bridge, adjacent to the abutment. It could be straight or splayed.
Weep-hole. Small holes often found at the base of masonry walls to permit the outflow of water which may have seeped into the infill of the structure. This is an important prevention strategy to avoid freezing and expansion within the infiill, which leads to bulging or tilting of spandrels and sidewalls.
Page last updated Oct.2020