Glossary of terms
Note on the use of the term – ‘Masonry’
Although sometimes used specifically to refer exclusively to building stone, here the word ‘masonry’ will be used in the broader sense, i.e. to refer generally to both brick and stone construction. When referring to brick or stone in particular, specific terms will be used, e.g. ‘stonework’, ‘stone arch bridge’, ‘brickwork’ and ‘brick arch bridge’.
Abutment: A body, usually of masonry, which provides the resistance to the vertical forces and the thrust of the arch.
Adobe: Regular body made of dried clay, usually incorporating straw to give it cohesion.
Antifunicular geometry: For a given set of loads, this is the geometry that results in an equilibrium state free from bending stresses, i.e. simply under axial section forces
Appraisal: Includes the range of activities involved with the evaluation of a bridge’s condition and performance, i.e. the gathering of existing data, inspection, investigation and structural assessment.
Archivolt: A projecting moulding which follows the curve of an arch above the extrados. Sometimes, the arch ring on the façade, or the shape of the arch curve.
Arch: A curved structural member capable of supporting vertical loads across an opening and transferring these loads to piers or abutments.
Arch barrel: The load-bearing part of the arch. It contains a single thickness of voussoir stones or several rings of brickwork or coursed random rubble.
Ashlar: Masonry consisting of regularly shaped blocks of stone square-dressed to given dimensions and laid in courses with thin joints.
Aspect ratio: The ratio of the span of a bridge to its width (its transverse axis).
Assessment: Here used specifically to imply the evaluation of a bridge’s structural capacity and performance, typically by one of a number of prescribed methods and possibly making use of proprietary software applications e.g. RING or ARCHIE.
Backing or Backfill: Material (usually low quality fill) used to give support behind a structure. For a masonry arch bridge, backfill material is placed in the spandrels between the arch barrel and the road surface and retained laterally by the spandrel walls and/or wingwalls. It normally consists of granular material, e.g. gravel or building debris, which may have been excavated for the foundations or is waste from the construction.
Barrel – see Arch barrel
Bedding plane: A plane of stratification in natural sedimentary stone.
Bed joint: Horizontal joint in masonry.
Bond: An arrangement of masonry units so that the vertical joints of one course do not coincide with those immediately above and below.
Bond types: Different bonds exist, including header, header/stretcher and stretcher bonds (depending on the kind of connection between the different rings). The most common type used in UK bridges is stretcher bond, in which there is no connection between rings.
Brick: A masonry unit comprising a shaped and kiln-fired block of clay or shale which can be used as an element the fabric of a bridge.
Bridge Engineer: A person responsible for the technical and engineering processes of bridge management, e.g. carrying out or making decisions regarding condition assessment, bridge capacity and serviceability, performance restrictions and requirements for maintenance, repair and strengthening.
Bridge strike: An incident in which a road, rail or waterborne vehicle, or its load, impacts on any part of a bridge structure.
Centring: Temporary structure on which an arch is supported during construction, normally made from timbers.
Common brick - a type of brick whose characteristics suit it to uses where strength and durability is not paramount.
Conservation: Work carried out to with the aim of maintaining or restoring the important features of a bridge, in particular the visible parts of its structure.
Corbel: Horizontal outward masonry projection (in brickwork usually constructed of headers) to provide an outstand from the normal line of masonry.
Corbelling: Structural form preceding the construction of arches, in which horizontal courses narrow progressively from the bottom upwards to create a structure capable of spanning an open space.
Elliptical arch: Indicating a ‘flattened’ semicircular arch, used to keep height reasonable, to reduce approach gradients, and to increase the width of gauge clearance below.
Engineering brick - a dense, strong and durable brick, often used for facing engineering structures.
Extrados: In an arch or vault is the surface the top surface of the arch barrel. The outer (convex) curve of an arch
Facing brick - a brick with suitable colour and durability for use in the exposed face of a masonry element.
Fill (or Infill): see Backfill
Haunch: The lower section of the arch barrel towards the springing
Header: A masonry unit laid with its longer dimension normal to the face of a wall or arch barrel, used to interconnects adjacent skins or rings of brickwork.
Hinge: A more or less local situation on which due to the formation of tensile openings, the structure can rotate as if it were an articulation.
Historic bridge: One that has some recognised historical value, through rarity or in terms of social, cultural or engineering heritage.
Hydraulic/Semi-hydraulic/non-hydraulic lime: A non-hydraulic lime is a more or less pure calcium hydroxide substance, used as a cement, which can only achieve a set through reaction with atmospheric carbon-dioxide. Hydraulic or semi-hydraulic limes also contain calcium silicates or calcium aluminates, and their set is to a greater or lesser degree assisted by chemical reaction with water.
Impost: The upper element of an abutment or pier which supports an arch barrel or other superstructure.
Intrados: In an arch or vault is the inner surface of the arch barrel, i.e. the inner (concave) curve of the barrel.
Keystone: The highest and last-placed stones in an arch. In the arch barrel of a bridge there are a series of keystones at the crown, across its width, which are often left projecting on side elevations.
Lime mortar: a mortar produced from pure limestone which relies upon gradual reaction with atmospheric carbon dioxide (‘carbonation’) to harden and develop strength.. Pure lime (also known as ‘fat’ or ‘non-hydraulic’ limes) produces a mortar that is typically weaker and more porous and permeable than impure limes with a degree of hydraulic (water-dependent) set or those containing Portland cement.
Maintenance: all the operations necessary to maintain it in a serviceable condition until the end of its life, comprising routine maintenance (routine work carried out with the aim of preventing or controlling deterioration, including inspection and monitoring activities) and essential maintenance (rehabilitiation works required to address specific inadequacies in function and performance, e.g. strengthening).
Masonry: The work of a mason, strictly referring to work in stone, however commonly used to refer generally to work in either brick or building stone, as it is here.
Mortar: Mix of one or more inorganic binders, aggregates, water and sometimes additions and/or admixtures for bedding ,jointing and pointing of masonry.
Natural Hydraulic Lime: a lime produced by burning of more or impure limestones with reduction to powder by slaking (the addition of water) with or without grinding. They have the property of setting and hardening under water, although atmospheric carbon dioxide contributes to the hardening process.
Parabolic arch: a very strong arch shape defined by the intersection of a cone and a plane parallel to the plane tangent of the cone. For uniform loads a parabola is theoretically an ideal arch shape because the line of thrust coincides with the centre-line of the arch ring.
Parapet: usually a vertical continuation of the spandrel wall; an upward extension of a spandrel wall above road surface level to protect those on and below the bridge.
Pattress plate: Load-spreading plate fitted at ends of tie-bars to restrain spandrels.
Performance: operation and/or functionality of a bridge or bridge element, in relation to the requirements of owners/operators/users.
Pier: has two definitions: (a) an intermediate support between adjoining bridge spans, or (b) a thickened section located at intervals along a wall to strengthen it.
Polycentric arch: an arch shape with more than one centre, ie. that is not defined as part of a single circle or curve.
Pozzolan: A cement additive comprising silica in reactive form, which can impart hydraulic set; can be either naturally occurring (e.g. volcanic ash) or artificially produced (e.g. brick dust or pulverised fuel ash, PFA).
Pulverised Fuel Ash (PFA): a waste product of coal fired power stations consisting of tiny spherules of reactive silica, sometimes used as a component in mortars and grouts.
Rehabilitation: Work that involves bringing features of a deteriorated bridge back into a satisfactorily functional state.
Ring: See Arch Barrel
Ring separation: Loss of bonding between adjacent rings (not necessarily a gap)
Rise: Vertical height of arch from springing level to the crown of the intrados.
Risk: a summation of the likelihood and consequences of an undesirable incidence
Roadway or Road surface: the upper surface of the bridge on which vehicular traffic runs, used here also to include the equivalent surface of bridges carrying rail traffic or waterways.
Roman cement: a term used to describe certain natural hydraulic limes produced by the calcination in a coal or coke fired kiln of limestone containing clay materials (principally silica and alumina). The name was apparently adopted in about 1800 because the red/brown colour and hardness resembled mortars of the Roman period.
Rubble masonry: The term describes many different types of masonry, the main types being random rubble (irregularly shaped stone elements, typically as it comes from the quarry) either coursed or uncoursed, and squared rubble (more regularly shaped stone), either coursed or uncoursed.
Saddle: A concrete slab cast over an arch to strengthen it or distribute loads upon it.
Scour: The removal of material from around structural supports by flowing water.
Segmental arch: Arch whose intrados comprises a segment of a circle which is smaller than a semicircle.
Semicircular arch: Arch with and intrados the shape of a semicircle, i.e. 180º, so that the rise is half the span.
Shallow arch: Arch in which the rise is smaller than a quarter of the span.
Skew arch: Arch where the longitudinal and transverse axes are not at right angles.
Skewback: Surface of an inclined springing.
Snap-through: Mechanism in which sufficient rotations take place at a hinge so as to produce an instability local failure, prior to the formation of a global hinge failure mechanism. This type of local failure can occur in highly confined arches and precipitates the global collapse of the structure.
Soffit: The underside of an element – in masonry arch bridges, equivalent to the intrados.
Soldier: Masonry unit laid with its longer dimension upright and parallel with the face of the wall, i.e. bedded on its smaller face.
Spalling: Loss of material from the face of a masonry unit, either through ‘flaking’ or delamination.
Span: Width of a void, which can be measured in different ways. Here it will be defined as the internal span, i.e. the distance between springings.
Spandrel: The area overlying the arch barrel under the road surface (or equivalent), occupied by fill material or voids, or occasionally hidden elements such as internal spandrel walls.
Spandrel wall: Masonry element that sits on the edge of the arch and that limits the extent of, and retains, the backfill. Sometimes ‘internal spandrel walls’ may be present at other locations on the arch.
Spandrel separation: Usually refers to lateral separation, in which the spandrel wall moves horizontally due to the action of applied loads, sometimes over the extrados of the arch and sometimes by forming a crack through the arch barrel close to its outer face. However, it could also be tangential separation, in which a crack tangential to arch forms at the contact between the arch and the spandrel walls.
Spreader beam: Load spreading strip over the length of the span and fitted at ends of tie-bars to restrain the spandrels.
Springing: Plane from which an arch springs, i.e. the junction between the vertical face of the abutment and the arch barrel.
Square spanning: Non-skewed arches.
Stretcher: A masonry unit laid with its longer dimension parallel to the face of the wall or arch barrel.
Stock brick – originally meaning one that is hand made using a stock mould, later coming to mean a large number (stock) of bricks all manufactured in the one locality, e.g. London stock brick.
Thrust line: The locus of the positions of the centroid of the compressive force within the arch. (the point on a given section where if you transfer the stresses, there is no bending moment but only axial force.)
Unit: A stone or brick element (masonry unit) that is laid, normally in mortar, to form masonry.
Vault: Either (a) the arched ceiling over a void, or (b) any space covered by arches.
Voussoir: Wedged-shaped masonry elements, generally with one convex face and one concave, used to make an arch or vault. Voissoirs can be flat or irregular in rubble construction.
Voussoir arch: Arch with one ring only, i.e. not a multi-ring arch.
Wing wall: A wall at the abutment of a bridge, which extends beyond the bridge to retain the earth behind the abutment.