Parts of a Bridge and Glossary

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Fig. 1

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Fig 2

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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 molding or banding surounding an arched opening.  

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 bulustrade.

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 with battered walls and included this in his specicifications. 

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 artifial island created by sinking a wooden frame filled with rubble, on to the required spot. 

ButtressA masonry block attached to a wall, often at regular intervals, serving to stregnthen it.

Centering.  A type of falseworks, 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 cenrering is ‘struck’ (removed). In fact, every voussoir in the ring must be place.  

Chamfer. Flat surface created by cutting off the edge of a squared stone voussoir.  Common on 16th century voussoirs.

Cofferdam. Originally, a Roman contrivance to create dry spce 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-wartertight 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 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 funcion.

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.

Countercourse. A second order of voussoirs.  

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. 

Drop Arch. A flat Gothic (pointed shape) arch in which the rise is less than half the span.

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. The soffit is the lower exterior surface of the barrel. It is not the same as the intrados. 

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. 

Falseworks. (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 on dry land,  to cope with potential spates and permit a flow escape during catastrophic floods.  Common on ancient bridges, floodarches 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. Piers were underpinned by foundations of Branders or Starlings.   

Freestone.  A type of sandstone or limestone with minimal grain, which can be cut or molded 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 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.

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 misconception 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. 

Orders. Courses of voussoirs in the arch.

Parapet. A low wall on either side of the decking, contiguous with the spandrels or side walls.

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. Arch (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. 

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.  

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.     

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 uncoursed 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. 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 parallel. There may be a reversal of flow at the pier front, a vortex, which also extends round the sides of the pier in a horseshoe. This rapidly erodes the foundations of the pier, eventially leading to damage and collapse. The arch spans then expand outwards leading to collapse of the arch. 

Segmental : The arch is 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. The plan is a parallelogram.  However, a false-skew does has voussoirs parallel to the abutments. This is only possible if the angle is less than 15° and even then, the prevailing pressure forces stones out at one corner.  Real skew constuctions have helicoidal, logararithmic or Cornes de Vache layouts requiring extensive stone cutting and shaping of the voussoirs. These bridges were not common until the railways arrived in in the 1840’s; roads can easily change direction to meet a river ar 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 rise/span ratio is a key characteristic of bridges and is a 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 and this may lead to characteristic longditudinal cracks in ild 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 horizantal course of masonry, usually narrower than other courses, and often protuding slightly.  Extending across the structure, it is purely decorative and serves to break up the large expanses of brickwork, or perhaps delineate the break beween spandrel and parapet.  

Voussoirs. ( Ring stones. Pend stones.) Individual wedge-shaped stones of a masonry arch. A ‘pend' was an arch in old Scots.  

Dec. 2012                                      Site last updated  Feb 2019