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. A solid masonry structure built to support the lateral pressure at both ends of a single arch. Multi-arched bridges have piers to support the middle arches but abutments and each end. (Figs 1,2,3)
Archivolt. An 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. (See Barrel, Arris, Soffit, Intra-dos and Extra-dos.)
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 (the intrados).
Ashlar. Finely dressed, perfectly cut, precisely shaped masonry stones; usually cuboid but 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. (see Rubble)
Baluster. Short pillars in series, supporting a rail, thus forming a balustrade.
Batter. A battered masonry wall is built with a deliberate inward slope towards the top. 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( Fig.4)
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.
Bedrock. The solid rock that lies below the soil and the subsoil. Engineers consider bedrock to be the material layer that has adequate bearing capacity for large structures.
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 (leaves) together. Rubble walls were usually built in two leaves with the space between them filled with small stones. Walls are usually 'bonded' every five or six feet. (Fig. 4)
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. ( see Starling)
Buttress. A masonry block attached to the side a wall, often at regular intervals, serving to strengthen it.
Catenary. The curve that an idealised hanging chain or cable assumes under its own weight when supported only at its ends. The lines of thrust of all arches take the shape of an inverted catenary curve; in order to confer stability, these lines of thrust should lie within the masonry itself.
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). This is true- but 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 :the arris (see above) has been sliced off to a varying degree thus softening the angle. Commonly seen on 16th century voussoirs.
Cofferdam. Originally, a Roman contrivance to create dry space for the underwater construction of piers (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. ( Fig 4.)
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. A primitive 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.
Course. A layer of the same squared units of masonry running horizontally in a wall. The units may be ashlar of squared rubble.
Counter-course. A second order of voussoirs.
Crown. The central highest part of an arch.
Cutwater. A 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.
Dead Load. Effectively, the weight of the bridge without traffic, traffic, passengers or goods ( see Live Load).
Dentilation. 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. A False Ellipse or three-centred arch is easier to construct (see below).
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). A temporary structure, usually of wood, used during construction to support spanning, in order to hold components in place until the structure is sufficiently advanced to support itself. ( see Keystone)
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.
Gallet. Small stones filling spaces in a random rubble facing, sometimes pushed into the mortar.
Gothic Arch. (Ogive, Pointed). A pointed arch formed of two arc segments meeting at the crown.
Header. An oblong squared stone laid with the head of the stone at the wall facing. ( see Stretcher). This is a form of ‘through-stone’. Roman bridges are known for their alternating header and stretcher stonework.
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.
Leaf. Masonry walls, seen in cross section, usually have more than one layer (or leaf). For example, a modern wall might comprise two layers of brick with a space in between. Plasterboard on the inside makes a third leaf. Ancient walls could have one, two or three leaves, most commonly, three: two stone and mortar leaves with a small-stone rubble filler leaf in-between them. There were always bond-stones at intervals ( Fig. 4) .
Length. This is the whole length of the structure, including all arches and abutments but usually excluding approaching ramps.
Live Load. The variable load on the bridge that shifts with precise
location and depends on the weight of traffic at any time as well as wind etc. The Total Gravity Load = Live Load plus Dead Load.
Mortar. A malleable paste which subsequently hardens, used to bind stones together whilst levelling out the bedding between them. Lime and sand mixture was the traditional material for this. Lime putty( lime and water) was used for the narrow space between ashlar blocks.
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. ( Fig.5)
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. Constructed 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. In cross section, there were usually three leaves in a wall (see above).
Rustication. Decorative treatment of finely cut ashlar. This is seldom seen on older Scottish bridges, because ashlar itself was not common on bridges prior to the 19th century. Rustication is a Renaissance decoration in which the facing surface of the stone is tooled to emphasise the square-block pattern. The simplest form is ’smooth-faced’ in which the edges are chamfered back to profile the interface between stones. A more pronounced ‘banded’ version of this is also seen. Droving, stugging or pitch-facing of each block face is common, frequently with flat margins. Diamond and Prism rustication is sometimes seen on 19th century bridges. Vermiculation is a pattern of dense irregular lines akin to worm tracks, also surrounded by flat margins.
Scabbling. The process of roughly squaring blocks of stone using a mason’s hammer. Usually done at the quarry prior to transport. (see Rubble)
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 remain 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 to scour.
Segmental Arch. An arch is the shape of a segment of a circle rather than a full semicircle.
Semicircular Arch. 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) The 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 arch-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 have 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; this is because roads can easily change direction to meet a river at right angles; whereas railracks cannot 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.
Spalling. A form of weathering a stonework, particularly at the joints, caused by pressure of uneven spacing. Salt ingress can also cause this by crystallising inside the mortar joint.
Span. The 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 than 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 and is a measure of the span. The 'rise' of an arch is the distance between the crown and the mid point of the springing line. The springing line may at water level, but on a stilted arch, sitting on abutments, it will be higher up.
Starling. An artificial island underpinning a pier. A common construction in English bridges. A cofferdam type enclosure was filled with rubble at each receding tide. In France, a concrete mix was used. ( see Brander)
Stilted Bridge. The arch is raised on the abutments or piers.
Stretcher. Oblong squared stone laid longwise in the wall-facing. ( see Header)
String Course (Band or Belt Course). A horizontal course of masonry, usually slimmer 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.
Three Centred Arch. This looks like an ellipse but in fact has three segmental sections moulded together. Two are identical, on either side, and a flatter, wider segmental shape in the middle. This shape is easier to construct than a true ellipse and implies a great deal less precise cutting of voussoir stones than on a true ellipse which varies gradually across the span.
Voussoirs (Ring stones. Pend stones). Individual wedge-shaped stones of a masonry arch. A ‘pend' was an arch in old Scots. (Fig.3)
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.
Weep-hole. Small holes often found at the base of masonry walls to permit the egress 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.
Wing-wall. (Fig.1) A retaining wall splaying out from the abutment, integral to the abutment wall, retaining the infill of the wider approach to the bridge. Their angle to the abutment provides a measure of buttressing.
Page last updated May. 23