Abutment The support at the ends of either a single-span bridge or a series of bridge spans. See also Pier.
Anthemion A Greek type of carved ornament based on flowers.
Aqueduct An elevated bridge-like structure carrying an artificial watercourse.
Arcade A row of free-standing arches supported on columns or piers or, in a blind arcade, placed decoratively against a wall and usually standing on pilasters.
Arch A vertically curved structural member that supports loads across a gap by resistance to axial compression along its length. The shape of an arch in a bridge gives its distinctive character. The most common bridge arch shapes are semicircular, pointed, segmental and semi-elliptical, although there are also parabolic steel-framed and concrete arches. Arches exert a lateral thrust against their abutments, this thrust increasing as the arch becomes flatter. In multi-arch bridges, the thrust from one arch is counter-balanced by the massiveness of the intermediate piers or by the thrust from the neighbouring arch.
Architrave The moulded lowest part of a lintel over a door, window or other rectangular opening.
Archivolt The projecting stone edging, often moulded, to an arch ring.
Arch ring The exposed edge of a masonry arch barrel that is visible on the side elevation of a bridge and is composed of stone voussoirs or vertically curved courses of brickwork.
Arris The corner edge along the line formed by the intersection of two surfaces.
Ashlar High-quality masonry consisting of smoothly dressed blocks of approximately square-faced stonework laid in regular courses with very thin joints.
Back span A span in a cable stay or suspension bridge between a main pier and the end of the bridge which, for a cable stay bridge, fully or partially balances the dead loads from the main span.
Bailey bridge A prefabricated portable bridging system invented by Sir Donald Bailey in 1940 for military use in the Second World War.
Balanced cantilever A structure consisting of a central pier or column with equal-length projecting arms on each side. In balanced cantilever construction, work proceeds simultaneously to the superstructure on both sides of a pier.
Balustrade An ornamental parapet consisting of a series of short vertical pillars (balusters), often moulded, standing on a base and supporting a handrail.
Banded (of a column) The encirclement of a column by a projecting strip, sometimes with a finish different from that of the column itself.
Barrel (of an arch) The curved, tunnel-like interior surface of an arched bridge comprising its working structure. The barrel of an arch is often constructed of a different (cheaper) material from that which can be seen on the elevation of the arch.
Bascule bridge A type of drawbridge in which the opening span pivots on a horizontal axis and is balanced by a shorter backspan, counterweight or both (from the French word bascule meaning seesaw). In castles such a structure is sometimes called a turning bridge.
Basket-handled A colloquial name sometimes given to a semi-elliptical or three-centred bridge arch.
Batter The slope of the face of a wall from the vertical.
Battlement A parapet of short alternating low and high sections, as in a castle.
Beam A straight structural member that spans, usually horizontally, across a gap to support loads through its resistance to bending.
Bearing A mechanism built into a bridge to transfer loads between structural sections, such as between deck and pier. One cause of the high cost of refurbishing or upgrading bridges is the difficulty of replacing worn, failed or undersized bearings. Where bearings have failed, often as a result of inadequate maintenance, additional stresses can be transferred into parts of the structure that were not designed to resist them, thereby endangering the structure.
Bent A two-dimensional frame of two or more legs placed at right angles to the line of the bridge it supports.
Blind arch A decorative arch placed against a wall.
Boom The top or bottom member of a truss or girder (sometimes called Chord).
Bowstring bridge A bridge with a vertically curved upper girder that meets a horizontal tie at the bridge’s support points, thus creating a profile similar to that of an archer’s bow and bowstring. The horizontal tie counteracts the outward lateral force from the ends of the arch, meaning that the bridge piers and abutments have to support a vertical load only.
Box girder A fabricated beam with a hollow square or rectangular cross section.
Bracing Secondary structural members, usually diagonal, which take tension or compression forces in order to stiffen a structure, for example against deformation from wind loading.
Bucket bridge A crossing that consists of a large wooden box, which can hold several people, supported by pulley wheels running along one or a pair of suspension cables spanning a river, that is then pulled across the gap by hand or a simple winch.
Butterfly bridge A bridge in which the deck is suspended below twin arches, with the plane of each arch and its filament of deck support hangers being inclined outwards from the vertical to form a V-shaped cross section.
Buttress A projecting support to a wall, usually at right angles to it, that helps the wall to resist lateral thrust.
Cable stay bridge A bridge in which the intermediate supports to a continuous beam are not from either piers beneath the beam or vertical hangers from a suspension cable above the beam but from diagonal ties, made from high-strength steel cable, connected directly between the point of support and a main pier or pylon that extends to some distance above the beam. The diagonal inclination of these stays imparts a compressive horizontal force into the bridge deck. In some early bridges of this type the stays were rods rather than cables.
Caisson A prefabricated circular or rectangular structure that is placed in water where a pier is to be constructed, the water then being pumped out allowing construction work to proceed in the dry. A caisson is incorporated in the foundations and remains a permanent part of the structure (cf. Cofferdam).
Cantilever A beam that is firmly fixed at one end and unsupported at the other in the manner of a bracket.
Capital The top of a column or pilaster.
Castellated The shape given to the top of a parapet wall, in imitation of the battlements on a castle, where alternate sections are higher and lower than each other.
Cast iron Iron smelted from iron ore and still containing carbon, that has been poured into moulds when in a molten state. The presence of the carbon makes cast iron brittle and unable to withstand tensile forces. It was replaced as a structural material by the introduction of wrought iron in the mid-nineteenth century.
Catenary The shape assumed by a cable of uniform self-weight when suspended freely between two points. It is thus the shape of a suspension bridge’s main cables before the deck has been hung from them. Although the cables in the completed bridge are sometimes still called the catenary, the actual final shape is a compound curve between that of a catenary and a parabola (qv).
Cement The finely-ground active agent in concrete, grout and mortar that, when mixed with crushed rock and/or sand, causes the whole to start hardening after water is added.
Centring The temporary structure, traditionally made of timber and sometimes called formwork, built to provide the support and necessary shape for an arch during its construction.
Chamfer The face resulting from the removal of an edge or arris between two surfaces at an angle to each other.
Chord The top or bottom member of a truss or girder (sometimes called Boom).
Clam bridge A primitive single-span bridge consisting of immense irregular ‘Cyclopean’ stone slabs spanning between abutments made from carefully placed piles of rock. The multiple-span form is called a clapper bridge.
Clapper bridge A primitive bridge consisting of immense irregular ‘Cyclopean’ stone slabs spanning between piers and abutments made from carefully placed piles of rock. The single-span form is often called a clam bridge.
Coade stone An architectural stoneware product made by firing a specially formulated mixture of pre-fired clay and sand. It was ideal for producing weather-proof decorative external features such as swags (which inside a building would have been made of plaster) that were cheaper than stone carvings.
Cofferdam A temporary water-excluding structure constructed in water, the water then being pumped out to allow construction work to proceed in the dry. When the permanent structure is completed the cofferdam is removed (cf. Caisson).
Colonnade A row of columns supporting a horizontal beam, or a series of arches.
Column An independent vertical structural member that supports loads by its resistance to compression.
Concrete A mixture of carefully graded stone and sand (the aggregates), cement and water which sets into a rock-like mass.
Concrete cancer Degeneration of concrete structures by a process properly known as alkali aggregate reactivity (AAR) or alkali silica reactivity (ASR). This occurs when silicates in concrete aggregate react with alkalis, such as de-icing salt or salts in seawater, to produce an expanding gel that leads to severe concrete cracking and loss of strength. Montrose Bridge in Scotland had to be demolished in 2004 as a result of AAR.
Continuous beam A rigid beam spanning between three or more consecutive supports without any structural break, thus giving it greater strength than identically sized beams spanning the openings singly.
Console A supporting bracket, similar to a corbel in function, but usually greater in height and with a scrolled outward face.
Coping The protective capping to a wall designed to shed rainwater.
Corbel A projection from a wall that supports a beam, arch or statue.
Corbel table A continuous corbel course usually supporting a parapet.
Corinthian The classical order of architecture typified by fluted columns with highly decorated capitals.
Cornes-de-vache A winding splay or chamfer given to the outside of an arch barrel by constructing the intrados of the main part of the barrel to a smaller-radius curve than that used on the face of the arch, the two curves being tangential at their midspan crowns. The amount of splay is therefore greatest at the abutments and diminishes to nothing at the crown.
Cornice The horizontal moulded projection that crowns part of a structure.
Course A single continuous layer of masonry (stone or brick) of the same thickness.
Covered bridge A bridge of timber protected from the weather by a wooden barn-like enclosure.
Crocket An ornamental carved projection on pinnacles and spires.
Crown The central, highest part of an arch.
Cutwater A pointed or rounded upstream or downstream extension to a bridge pier at water level to smooth the flow of water past the pier.
Cyclopean Masonry work consisting of immensely large irregular pieces of stone.
Dead load The self-weight of a structure and any loads permanently supported by it. See also Live load.
Deck The upper structural part of a bridge, the top surface of which forms the roadway.
Deck truss A bridge in which the deck is supported on the upper members (booms or chords) of a truss or girder bridge.
Dentil One of a series of small, usually square, projections from a wall that support a cornice.
Doric The classical order of architecture typified by fluted, sometimes baseless, columns with simple capitals.
Drawbridge A beam structure that is pinned at one end of its span so that the free end can be raised to prevent passage across the bridge or to enable large vessels to pass beneath.
Entablature The part of a classical structure standing immediately upon column capitals and including the architrave (the main lintel or beam), the frieze and the cornice.
Expansion joint A mechanism built into a bridge to allow deck sections to expand and contract as a result of temperature changes without inducing additional horizontal forces at the tops of piers.
Extrados The line (usually curved) followed by the outer edge of an arch ring. In a bridge with stepped extrados the outer edge of each voussoir is not cut to a radial curve but finished with vertical and horizontal faces to fit into the coursed masonry of the spandrels.
Extradosed bridge A type of post-tensioned box girder bridge in which the prestressing cables emerge from the deck and are anchored to low pylons located above the supporting piers. The bridge profile is thus superficially similar to that of a cable stay bridge, except that the prestressing tendons are at a shallower angle than cable stays would be.
Eyebar A flat metal bar, usually rectangular in cross section, enlarged at each end to accommodate holes allowing adjacent eyebars to be connected together by pins through these holes.
Fan An arrangement of several tension members in a cable stay bridge or struts under compression, for example in a timber bridge, radiating out from a common point in the manner of a lady’s fan.
Fatigue The reduction in tensile strength of a structural material following repeated application and removal of heavy loading.
Finback bridge A bridge in which the main supporting structure takes the form of a rectangular beam built above the centreline of the bridge deck, the overall cross section thus appearing like an inverted T.
Finial A vertical ornamental feature that terminates a gable, pediment or similar.
Fink truss A structure in which a series of inverted king post trusses overlap, with the diagonals from one truss ending at the top of the vertical king post of the adjacent truss.
Fish belly beam A beam in which the top edge is straight but the depth increases from the ends to the middle to give the bottom edge a sagging curve (the opposite of Hog-backed).
Flange The wide top or bottom horizontal sections of an I-shaped joist, truss or girder that are separated by the vertical web.
Fluting The decorative vertical grooving around the circumference of a classical-style column.
Folly A costly and unnecessary structure.
Four-centred arch A pointed arch, also known as a Tudor arch, in which each half span is formed by two arcs, an outer one of small radii and an inner larger arc, which meet together at a common tangent point.
Girder A made-up beam member either with a solid web (a plate girder) or a trussed web consisting of vertical and/ or triangulated members. See also Lattice girder, Vierendeel girder and Warren girder.
Gothic An architectural style, typified by pointed arches, common from the early thirteenth century.
Grout A rich, binding slurry of cement and water that can be poured or pumped into a structure for filling small unwanted gaps between beams and the like.
Half-through bridge A bridge in which the deck is located about mid-way between the upper and lower members (booms or chords) of a truss or girder bridge.
Helix A curve like a spiral but with a constant radius rather than the reducing radius of a true spiral.
Hog-backed girder A girder in which the bottom edge is straight but with a vertically curved upper member that does not meet the lower boom as in a bowstring girder (the opposite of a fish belly beam).
Hyperbolic paraboloid A three-dimensional, doubly curved surface looking like a saddle, with the special property that every point on the surface lies on two separate straight lines. A model can thus be constructed with string. It is mostly used in roofs.
Impost The top course of a pier or abutment, sometimes with moulded decoration, from which a semicircular or semielliptical arch springs. See also Skewback.
Indulgences Remission of time spent in purgatory granted by the medieval church in return for funds for the building, repair or maintenance of bridges.
Intrados The curved line followed by the inner edge of an arch ring.
Invert The paved surface beneath an arch over which people, traffic or water will move.
Ionic Classical order of architecture typified by slender fluted columns with capitals decorated with scrolls (volutes).
Jack arch A small arch, usually of brick, spanning between the bottom flanges of adjacent steel joists to support a deck between them.
Jagger Early user of packhorse bridges, especially in mining areas.
Jettied A form of medieval building construction in which the first floor cantilevers out beyond the ground floor.
Jetting A technique for installing piles in sandy ground by using pressurised air or water to help the pile driving.
Keystone The central stone in a ring of voussoirs forming an arch. Structurally, the keystone is no more important than any other stone in an arch ring (as with the links in a chain) but, being at the crown of the arch and the last to be laid by being driven into place, is sometimes made visually more dominant.
King pier An enlarged pier built at intervals in a lengthy viaduct to increase stability during construction.
King post truss A triangular framework, mainly used for roofs, in which a vertical post connects the centre of the horizontal tie beam to the apex where the sloping rafters meet.
Knee brace A short diagonal strut between the underside of a beam and its support to increase the stiffness of the beam.
Ladder beam bridge A bridge with a composite deck of concrete and steel where concrete slabs span longitudinally between steel crossbeams which themselves span between two longitudinal steel edge beams.
Lambswool finish Surface treatment of stonework involving the close scribing of parallel lines.
Lamination (of timber) The bolting or gluing together of several planks to make a single timber beam larger and stronger than could be formed from a felled tree.
Land bridge A structure, invisible from above, that carries parkland across it without a break.
Lattice truss or girder A fabricated structural beam in which the web space is made up of individual intersecting diagonal members in the form of overlapping Xs.
Lenticular bridge A bridge with curved top and bottom booms in the shape of an optical lens. The top boom is in compression and the lower one in tension, these forces cancelling each other out where the booms meet at the abutments, thus transferring vertical loads only into the abutments.
Lintel A horizontal beam spanning across a door, window or other rectangular opening and supporting the wall above.
Live load A load that can be temporarily applied to a structure and then removed again, such as from moving people and traffic, as well as occasional static loads such as snow. See also Dead load.
Locked-coil cable A helically-twisted wire cable made up of concentric layers of individual wires, alternate layers being wound in opposite directions, and with the outer layers consisting of Z-shaped wires. When such a cable is put under tension the coils try to straighten out, causing the individual wires to tighten against each other thus sealing the inside of the cable. Locked-coil cables, manufactured to the exact lengths required, are usually used for the stays of cable stay bridges.
Machicolation A parapet supported on corbels and projecting forward from its support wall with openings in the floor between the corbels.
Masonry bridge construction The diagram on the following page shows the main parts of a stone bridge. All the terms shown have definitions included in this Glossary.
Merlon One of a series of higher parts in a battlemented wall of alternately higher and lower sections.
Network arch A bowstring bridge in which the hangers supporting the deck from the arch are not vertical but are inclined, crossing each other at least twice in a single plane, thus making the structure act somewhat like a lattice truss.
Niche A recess in a wall or pier, usually semicircular in plan, sometimes containing a statue.
Oculus A circular, decorative recess in a wall, usually with a solid back (blind), ringed with a moulded rim.
Ogee An S-shaped continuous double curve that is part convex and part concave.
Order In a medieval masonry bridge, the number of concentric rings of voussoirs that do not share the same plane of elevation. In architecture, the main divisions of classical style – Corinthian, Doric, Ionic and Tuscan – distinguished most easily by their different types of column head decoration.
Orthotropic deck A steel plate deck stiffened and supported by two sets of steel joists or other sections, at right angles to each other, all welded together into an integral structural unit.
Overbridge A bridge across a road, railway or canal to carry another thoroughfare over it.
Palladian Describing a structure based on the drawings of Andreo Palladio (1508–1580), whose designs were often inspired by the buildings of ancient Rome, especially those of Vitruvius (c.75–c.15BC).
Panelled girder A type of trussed girder in which the space between the top and bottom booms is divided by vertical members into a series of panels that, except for Vierendeel girders, are braced internally by additional diagonal members.
Parabola A curved line defined mathematically as that described by a point that is the same distance from a fixed straight line (the directrix) and a fixed point (the focus). This is also the shape assumed by a suspended cable of negligible self-weight when loaded with a weight distributed uniformly along the horizontal distance between the suspension points.
Parabolic arch An upright arch constructed as a mirror image of the sagging line of a parabola (qv) and thus the most economical arched shape to support the evenly distributed weight of a separate horizontal bridge deck.
Parapet The edge wall running along the length of a bridge deck.
Patera A decorative, low relief, circular ornament.
Pattress plate A circular, S-or X-shaped load-spreading device, sometimes seen on the spandrel wall of a masonry arch bridge, that is linked by a tie bar to a similar one on the opposite side of the bridge to prevent the walls bowing outwards under the pressure of the spandrel fill.
1. Quoin
2. Corbel
3. Refuge
4. Dentil
5. String course
6. Coping
7. Parapet
8. Spandrel
9. Balustrade
10. Spandrel with radiating masonry
11. Keystone
12. Steeped arch
13. Pylon
14. Oculus
15. Pediment
16. Niche
17. Pilaster
18. Skewback
19. Arch ring
20. Pier with pointed cutwater
21. Impost
22. Voussoir
23. Arch
24. Rib
25. Springing line
26. Arch ring
27. Abutment
28. Buttress
29. Wing wall
Pediment A triangular or segmental low-pitched gable above an archway or similar opening. In a broken pediment the central part at the top of the gable or part of the horizontal base is omitted.
Pier The intermediate support between two spans of a bridge (see also Abutment). Also a seaside structure built mainly to provide landing stages and other leisure facilities for holidaymakers.
Pilaster A shallow decorative column attached to a wall.
Pile A timber, steel or concrete post inserted deep into the ground to carry loads that cannot be resisted by the ground either at its surface or at a shallow depth.
Pin joint A connecting joint between structural members that allows the members to rotate around the connection, thus making it impossible to transfer bending moments between them.
Plate girder A fabricated structural member made up of a solid vertical web with flanges attached to the top and bottom edges.
Plinth The square base of a column or the projecting base of a wall or balustrade.
Pointed arch An arch in which the curves on each side meet at the apex without this being a common tangent point.
Pontage Authority, by ancient custom or royal or parliamentary grant, to levy a toll for crossing a bridge in order to raise money for its maintenance or repair.
Pontoon bridge A bridge (often only temporary) constructed by anchoring boats across a river and laying a continuous roadway over and between them.
Portal frame A structural framework consisting of a beam rigidly connected to two upright posts.
Porte-cochère A covered entrance-way to a grand house allowing coach passengers to get on and off in the dry.
Post-tensioned See Prestressed concrete.
Precast concrete Concrete structural elements such as beams that can be manufactured in controlled conditions off site.
Prestressed concrete Concrete in which tensile stresses induced by the self-weight of the structure, together with its expected load of vehicles and people, are reduced by compressing the concrete by means of high strength steel wire or cables. This prestressing is applied either by stretching the wires before the wet concrete is poured around them (pre-tensioned), or by passing wire cables through ducts formed in the concrete when it is poured, stretching these after the concrete has fully hardened (post-tensioned), then filling the duct with grout.
Pultrusion The process of manufacturing thermoplastic materials by pulling a mixture of reinforcement fibres impregnated with resin through a heated die.
Pylon In architecture, a plain tapering elongated column like an obelisk, used for decorative effect only. The word is also used for the above-deck extension of a pier in a cable stay bridge, to which the diagonal cable stays are anchored.
Quatrefoil A Gothic-style opening in stonework consisting of four small arcs meeting at pointed cusps.
Quoin A corner, or the line of bricks or stones, often decorative, that forms a vertical corner between two walls.
Raft A wide foundation platform, often made of timber, that sits on poor load-bearing ground and on which a structure such as a pier is then erected.
Refuge An alcove in the line of a bridge parapet, usually above a cutwater, to give pedestrians safety from passing traffic.
Reinforced concrete Concrete that contains steel bars or mesh that resist the tensile stresses in, for example, a beam supporting loads through its resistance to bending, thus preventing weakening cracks developing.
Rib The part of a masonry arch that projects beneath the main barrel of the arch and was usually constructed first to economise on centring, the remaining part of the barrel being constructed by spanning stones between adjacent ribs; or an arched frame in metal or concrete.
Ribbon bridge A type of suspension bridge in which the roadway or footway is carried directly on top of the suspension cables rather than being hung beneath them.
Rise (of an arch) The vertical distance between a horizontal line joining the springing points of an arch and the underside of its highest point.
Rod stayed bridge An early form of stayed bridge in which the stays were metal bars rather than cables.
Rolling lift bridge An opening bridge in which the main span is balanced by a shorter back span in the form of a quadrant which, as it opens, rolls backwards on tracks in the manner of a rocking chair.
Rubble Masonry work consisting of differently sized stones laid without any pattern (random rubble) or in courses (coursed rubble).
Rusticated masonry Coursed stonework laid with deeply recessed V-shaped joints.
Saddle A strengthening layer, these days usually of reinforced concrete, placed above the extrados of an arch and often constructed to increase the loading capacity of old stone arch bridges. Also, the curved member at the top of a suspension bridge tower over which the main cables pass.
Scour The erosive action of water currents on the bed or banks of a river.
Screed A thin levelled finish of sand:cement mortar laid over rough concrete.
Segmental arch An arch in which the shape is formed from a circular arc smaller than a semicircle.
Semi-elliptical arch An arch in which the shape is a curve joining points located a constant distance from two fixed points, called foci. In a true semi-elliptical stone arch every voussoir in each half arch has to be cut individually and precisely. In order to simplify the stone-cutting and the setting out of the arch, many apparently semi-elliptical arches are in fact three-centred.
Skew The obliqueness between the centreline along the length of a bridge and a line at right angles to the abutment face.
Skewback The angled stone or other structural element from which a segmental arch springs. See also Impost.
Soffit The exposed underside of a beam or slab.
Span The distance between the supporting points of a structure. In stone and brick bridges the span is measured between the faces of the piers or abutments, whereas for steel and concrete structures the span is usually measured between the bearings that are inset behind the abutment face or on the centre of intermediate piers.
Spandrel The roughly triangular-shaped vertical area between the outside of an arch ring (the extrados) and the underside of the bridge deck structure.
Split bridge A bridge over a canal built as two separate half-cantilevers separated by a small gap. This allows a rope by which a barge is being towed to be passed through the central split without the horse having to be unhitched.
Springing The line along which the curved underside of an arch barrel meets the face of a pier or abutment.
Squinch arch An arch built across a corner to carry some superstructure, often one of a series of arches, each corbelled out from the one beneath.
Starling An artificial island contained by contiguous timber piling and formed of stones and gravel on which a pier will be constructed, or created after a pier has been built, to protect it from scour.
Stayed bridge A bridge in which the deck is supported by diagonal tension members (cables or rods) from a pylon above deck level.
Steel An alloy of iron and carbon with small amounts of special metals such as chromium, manganese, molybdenum and vanadium. Steel replaced cast and wrought iron as a structural material at the end of the nineteenth century.
Stepped arch An arch of stone voussoirs in which the outside edge of each voussoir is not cut to form part of an extrados curve but is shaped with a right-angled outside top corner, the vertical line abutting a horizontal course of masonry in the spandrel, the next course of spandrel masonry being continued above the horizontal top of the voussoir.
Stilted arch Arch in which the springing points are placed relatively high up on the supporting piers.
Strain The alteration in the shape of a structural member resulting from the application to that member of a compressive, tensile, torsional or shearing force.
Stress A force per unit area on a structural member, the result of which is strain.
String course A single course differentiated by colour, material, alignment or finish from the face of a vertical surface; typically a narrow projecting course, sometimes moulded, that serves to separate different parts of a wall.
Strut A structural member that contributes to a truss, girder or structural framework by resisting compression forces.
Stucco A type of plaster, normally painted, used as a hard-wearing external finish to lower quality brickwork.
Suspension bridge A bridge in which the deck is supported by hangers from cables that are suspended between tall towers and anchored firmly into the ground at each end or, occasionally, into the ends of the deck structure itself (self-anchored). See also Ribbon bridge.
Swag A carved decoration in the form of a length of drapery or foliage supported at the ends and sagging between.
Swing bridge A type of opening bridge in which the opening span rotates in a horizontal plane by pivoting on a vertical axis.
Three-centred arch An arch in which the shape is formed by three arcs, a central large radius arc and two outer ones of smaller radius, which meet at common tangent points. In order to simplify construction, many bridges that appear to be semi-elliptical in shape are in fact three-centred (with the smaller radius arcs being tangential to the vertical at the springings). Also, some segmental arches, instead of springing directly from a skewback, start with a very small radius arch off the abutment face.
Through truss or bridge A bridge in which the deck is supported on the lower members (booms or chords) of a truss or girder bridge.
Tie A structural member that contributes to a truss, girder or structural framework by resisting tensile forces.
Transporter bridge A fixed high-level structure that carries a suspended moving low-level platform at road level.
Transom A horizontal cross beam in a framework.
Trapezium A four-sided geometric shape with one pair of opposite lines being parallel.
Trefoil A Gothic-style opening in stonework consisting of three small arcs meeting at pointed cusps.
Trenail A cylindrical hardwood pin driven into preformed holes to connect together separate pieces of timber in structures exposed to the weather.
Trestle A lightweight frame of two or more legs supporting a further structure. A bent is a two-dimensional trestle.
Triglyph An architectural feature, derived from classical Greek temples and representing the ends of timber roof beams, now consisting of a stone block incised with three vertical grooves repeated in a series set into the frieze part of an entablature.
Trunnion The horizontal axis of rotation for a modern bascule bridge.
Truss A rigid fabricated structural framework, made up by connecting together smaller members carrying compressive or tensile forces only (struts or ties) and usually arranged in the form of triangles, that acts as a beam or girder.
Tudor arch A four-centred arch in which the two arcs that meet at the pointed apex are nearly flat (sometimes called a depressed arch).
Tufa A lightweight porous limestone mainly used for building exotic structures.
Turning bridge A particular type of drawbridge used in medieval castles that pivoted on a central horizontal axle.
Turnover bridge A bridge that carries a canal towpath from one side of a waterway to the other.
Tuscan The classical order of architecture typified by unfluted columns with little or no decoration to the capitals.
Two-centred arch A pointed arch in which the curves on each side meet at the apex without this being a common tangent point.
Underbridge A bridge under a road, railway or canal to carry that thoroughfare over an obstacle. A canal underbridge is called an aqueduct.
Vault An arched roof of stone or brick.
Vermiculated A masonry finish in which the stone surface is carved to give the appearance of worm tracks.
Versine The vertical distance a catenary cable sags at the midpoint between its supports.
Viaduct A generally lengthy road or railway bridge, usually flat or at a slight constant gradient, with a series of relatively short spans.
Vierendeel girder A made-up girder, the web of which has no diagonal members and consists solely of members at right angles to the top and bottom chords and connected rigidly to them.
Voussoir A wedge-shaped piece of accurately dressed masonry with opposite faces cut to slopes normal to the intrados curve and which, with its neighbours, makes up the structural part of an arch. For semicircular or segmental arches the voussoirs can be interchangeable; with true semi-elliptical arches, every voussoir in each half arch must be individually shaped.
Warren girder A made-up girder, the web of which consists solely of non-overlapping diagonal members, successively sloping in opposite directions, giving a repeated W-shaped elevation.
Web The central part of an I-shaped joist, truss or girder that separates the flanges.
Weir A low-level dam across a river with water flowing over its full length. Many old bridges are just upstream of weirs, built to reduce the speed of the water flowing under the bridge and thus to lessen the risk of scour damage to the pier and abutment foundations.
Wing wall A wall behind an abutment to support the sides of a bridge approach embankment or ramp.
Wrought iron A type of iron that has been specially worked to reduce its carbon content to a minimum. It was first used as a structural material in the mid-nineteenth century, taking over from cast iron, before being replaced by steel. It is able to resist tensile forces (unlike cast iron) but suffers from fatigue and is more expensive.