An explanation of some terms used in structural engineering, architecture and construction in the UK.

An explanation of some terms used in structural engineering, architecture and construction in the UK.

Fr.:- French equivalent terms, nouns are m(asculine) or f(eminine).

Fr.:- French equivalent terms, nouns are m(asculine) or f(eminine).

	A A B B C C D D E E F G H I J K L M N O P Q R S T U V W Y top	A: Access chamber:  Underground chamber enabling access to drains or other underground services. Acre:  Unit of land area in the Imperial system; 4840 square yards, or the equivalent of a rectangular field one chain wide and one furlong long, approximately 4047 square metres or 0.4047 hectares. Acrow: A telescopic prop much used as a temporary support in construction. Named after the American manufacturer who first introduced them to the UK. Additive: Chemicals added to cement based products (concrete, mortar, render, screed etc) to impart various desirable properties such as to increase or reduce curing time, increase strength, enhance workability and so on. The amount of additives should be watched carefully since in excess or combination they can have undesirable effects. Aggregate:  The stones and sand (coarse and fine aggregate respectively) used as a filler in concrete, asphalt etc. Air conditioning: Originally, a system by which fresh air is drawn from outside the building and brought to an acceptable condition in terms of temperature and humidity before being introduced into the building. The name is often also applied to chillers with no air handling, drying or heating capacity. Aircrete: A lightweight aerated cement-based material from which easily handled high insulating building-blocks are made. (Trade name.) All-in ballast: Ballast suitable for making into concrete without the addition of any other aggregate. Angle: Steel angle: a steel section whose cross-section is L-shaped. If the vertical and horizontal legs of the 'L' are the same length it is called an equal angle, if different, an unequal or odd leg angle. Angles are also available in other metals.   An amount of rotation. The measurement of angles using 360 degrees in a whole circle, with each degree divided into 60 minutes of 60 seconds each, is of very great antiquity, going back to the Babylonians who used a number system based on 60s rather than tens. Arch: A basic form of masonry construction dating back millenia. Brick arches are found spanning over window and door openings in Victorian and older buildings; their disadvantage is that they exert horizontal thrust at their bearings, which sometimes leads to distortion in poorly designed or maintained arches. Architrave: Timber moulding around a door frame or similar. Arris: A sharp corner at the junction of two planes or surfaces. Arris rail: Timber of triangular cross section (made by cutting a square section diagonally), used for fence rails and forming fillets at the junctions of flat roofs and walls. Ashlar:  Smooth sawn stonework used in a wall. Axed arch: A brick arch in which the bricks are cut (traditionally with an axe) to a wedge shape. The mortar joints are of even thickness. As opposed to a rough arch. B: Back addition: Traditional terraced housing originally comprised rooms between the front and rear external walls. When 'indoor plumbing' became the rage, extensions were built at the back of the house to contain the bathroom, wc, kitchen and scullery. The rear wing of a house is still called the back addition, even if it was built at the same time as the rest of the house. Ballast:  Mixed size aggregate. Batten: A small timber such as those used to support roof tiles. Beam: A horizontal member that carries vertical loads along its length. It would traditionally have been timber (the word originally meaning "tree trunk") but a modern beam might more often be reinforced concrete or steel. (Fr. poutre, f)   A steel component designed for use as a beam; "Universal Beam". Bench-mark: A levelling base point of known level. The Ordnance Survey has set up bench marks around the UK. Contractors often establish 'temporary bench marks' ('TBM') at convenient points around the site. Bending moment: The bending force in, for example, a beam. The units of bending moment are those of force x distance, for example, kiloNewton-metres. Berm: An earth bank left against a retaining wall during excavation, until it is propped. Bessemer converter: A kind of steel-making plant, no longer in use. Bill of quantities (BOQ): A list of all the quantities of each component and operation required in a construction project. The BOQ enables all the tenderers to price exactly the same work, and makes it simple to work out the value of the work done at any time during the job. For small jobs the benefit of a BOQ may be outweighed by the cost of producing it. Blinding: A layer of concrete covering the ground so that steel reinforcement can be laid out without becoming contaminated. Block: Building unit of a regular size usually made of solid or aerated ("aircrete") concrete. Blockwork: Built with blocks. Bolt: Threaded fastener used (with a nut and washers) for connecting building components, particularly steel and/or timber. Bond: The arrangement or pattern of bricks (or other masonry units) in a wall. Each unit should overlap the unit below by at least one quarter of a unit's length, and sufficient bonding bricks should be provided to prevent the wall splitting apart. Common bond patterns are Flemish, Stretcher, English and English Garden Wall. Bonding plaster: A proprietary type of plaster with good adhesive properties. It must be used with care as it is hygroscopic, i.e. it will readily absorb atmospheric or rising moisture. Box gutter: A timber gutter lined with lead or some other waterproof material. (Fr. chèneau (m) encaissé). Brace, Bracing: Diagonal members (or rigid membranes) providing rigidity to a structure.   Bressemer, Bresumer etc.: A timber lintel flush with the surface of the brickwork above it. Brick: Building unit of a regular size usually made of baked clay. Can also be calcium silicate or concrete. The standard size of metric bricks in the UK is 65 x 102.5 x 215mm, designed to be used with a 10mm mortar joint. The equivalent theoretical size of imperial bricks, used with a 3/8 inch joint, is 2 5/8 x 4 3/16 x 8 5/8 inches. Clay bricks are of course of great antiquity as evidenced by archaeology and the bible. (Fr. brique, f). Brick guard: Steel mesh panel used on scaffolding to make sure that loose bricks cannot fall off the scaffold. Brick tie: A metal or plastic component to tie together the two leaves of a cavity wall. Older galvanized ties tend to rust away and have to be replaced. Bricklayer: A skilled trade which requires years of training and practice. (Fr. maçon, m). Brickwork: Made of bricks. (Fr. maçonnerie, f). Bucket-handle pointing: Recessed in the half-round shape of an old-fashioned metal bucket handle. Building Control: The first Building Control was introduced, in London, after the Great Fire (1666) when District Surveyors were engaged to enforce the Building Regulations which prevented the spread of fire from house to house – the Regulations had existed before but had often been ignored. The system now covers the whole UK and includes rules on most aspects of building as it affects public safety and health, enforced by Building Control Officers. See links for details. Not to be confused with Town Planning. Building services: Plumbing, electrical wiring, ventilation, gas supply and other support systems in a building. C: Calcium silicate bricks: Smooth bricks made by compressing and heating a mixture of sand, or ground flint, and lime. Popular in the mid 20th century but less used now, because of their tendency to shrink. Camber: The rise in the middle of a roadway for drainage, or the similar shape given to a beam so that it will become level when loaded. Cantilever: Overhanging beam, roof or floor. Casement: A window which is hinged rather than sliding. Cast iron: A brittle material no longer much used in structural engineering. Cavity tie: See brick tie. Cavity wall: A wall consisting of two leaves or skins of masonry, seperated by a cavity to enhance water resistance and thermal insulation. A form of wall construction known but rarely used in Victorian times but which came into common use in the 1930s. (Fr. mur (m) à double paroi). Cement: A powder which when mixed with water forms a paste that hardens with time. Portland Cement was first patented by Joseph Aspin in 1824 and is known as hydraulic cement, because it will set under water. Cement is mixed with sand to make mortar or render, and with larger stones added it is known as concrete. The sand and stones are there to reduce the shrinkage to which Portland cement is subject and to reduce the amount of cement needed. There are various grades: the usual one is called Ordinary Portland Cement (OPC); others commonly used are rapid hardening and sulphate resisting. Cement mixer: Mechanical device consisting of a rotating drum with fixed paddles inside, used for mixing cement with aggregate and water to produce concrete, mortar, or any other cement-based mixture. Centring: Temporary supports used when building an arch. Chain: Surveyors' unit of length in the Imperial system. Gunter's chain, named after its inventor, comprises 22 yards or 66 feet, approximately 20.117 metres. Gunter's chain is useful for deriving areas in acres. The lesser-known Engineer's chain, 100 feet long, was used for measuring linear distances, along roads for example. Channel: A structural steel component which is C-shaped in cross section. Characteristic strength: The strength at which a member tested would fail, normally with 95% confidence. Cill: Alternative spelling of sill. Circular hollow section: A structural steel component in the shape of a round tube. Cladding: The seperately-applied exterior finish of a framed building. Clamp: See cramp. Classical orders of architecture: The classical orders are styles of building originating from the construction of temples in ancient Greece and Rome. Orders are defined by their varying styles of column, although the orders also include information on the proportions of the building. The Greeks originally had three orders: the Doric, Ionic and Corinthian. Doric is the simplest, Ionic more elaborate, and Corinthian more decorative still. The Romans added the Tuscan and Composite orders which are respectively plainer and more highly decorated than the Greek orders. Cleat: A steel plate or angle with holes for bolting, for connecting the components of a steel frame together. Coarse aggregate: Any aggregate larger than fine aggregate. Gravel. Available with a maximum size of 10, 20 or 40mm. Collar: A horizontal timber joining two opposing rafters together. Collateral warrantee: A legal agreement between a developer and a building contractor or designer, allowing the contractor or designer to be made responsible to a third party, such as a finance provider or a purchaser, for the execution of their duties. Common rafter: A normal rafter, which extends all the way from wall plate to ridge, as opposed to a jack rafter. Compasses: An instrument for drawing arcs and circles. Not to be confused, incidentally, with a compass (in the singular) which is a magnetic instrument for finding North. Competent person: Person with sufficient knowledge of the specific tasks to be undertaken and the risks which the work will entail, and with sufficient experience and ability to enable them to carry out their duties in relation to the project, to recognize their limitations, and to take appropriate action in order to prevent harm to those carrying out construction work, or those affected by the work. (Construction Design and Managment Regulations 2007) Composite order: One of the ancient classical orders of architecture, introduced by the Romans. Its capital combines the volute scrolls of the Ionic with the acanthus foliage of the Corinthian. Compression: The pressing force experienced in a column or in the top flange of a beam. Computer aided design (CAD): The type of computer program with which technical drawings are prepared. The market leader is AutoCAD but there are others. Concrete: An artifical stone-like substance obtained by mixing large and small stones and sand with cement and enough water to permit full hydration and make the mix workable. Concrete (like the stone minerals from which it is made) is strong in compression but weak in tension. Roman concrete was based, not on Portland cement, but on a 'pozzolanic' mix, made from volcanic ash and incorporating ground-up bricks and tiles. (Fr. beton, m). Concrete pump: A machine for transporting concrete down a delivery pipe. May be truck mounted or static. Contract: Building contracts may legally be formed verbally. Usually however a written contract should be used. There are various standard forms of contract, such as those provided by the Joint Contracts Tribunal and the various engineering institutions. Contract administrator: Many forms of building contract specify a Contract Administrator to manage the contract on behalf of the Client. It may be the architect, the engineer, or a specialist such as a project manager. The CA's main duty is to specify how much the contractor is due to be paid at each stage. Contract documents: The contract drawings, bill of quantities, specifications, and any other documents referred to in the contract. Contract drawings: The drawings on which the contract is based. Coping: Protective capping on the top of a parapet or free standing wall. Corbel: Projecting brick or masonry courses; from Norman-French meaning 'crow' after carved stone projections used in medieval times to support roof trusses. Corinthian order: The most elaborate and decorated of the three ancient Greek orders of architecture, its capital is carved in imitation of the growth of acanthus leaves. According to Roman writer Vitruvius, a young lady of the nobility in Corinth died, and her nurse placed a basket containing her belongings on top of the grave, with a roof tile on top to protect it. An acanthus plant grew right under the basket and its shoots curved and rolled around the corners of the tile. A passing architect noticed this and copied it in stone. Corrugated iron: (Corrugated galvanized iron). Iron (or for the last hundred years at least, steel) sheet formed into a ridged shape, used for roofing and cladding. Coupler, coupling: A device for mechanically joining two linear components like pipes, scaffold tubes, or a drill bit with an extension. Course: A layer of bricks or blocks in a wall. Cramp (also clamp): Metal component built into masonry to join it to another member, for example a window frame ('frame cramp'), or to join two masonry units together. Crane: Lifting device which can be fixed or mobile. Crippled: Of joists, doubled-up to form a trimmer. (American term.) Cure: The hardening of concrete and other cement products. Curing requires a certain range of temperature (more than 6C but not enough to cause thermal stress) and sufficient internal water to combine with the cement. D: Dado: A timber moulding fixed to the wall at waist level. Damp proof course (DPC): An impermeable material built into a wall near the ground to prevent rising damp. Types available include lead-based, bitumen-based, or plastic-based. Two courses of impermeable engineering bricks can also be used. The DPC must be at least 150mm above the external ground level. Damp proof membrane (DPM): Usually heavy duty polythene, incorporated within floors built on the ground to prevent rising damp. Dead load: The weight of the materials which form a permanent part of the structure, as opposed to imposed load. Deal: Softwood; a standard piece of softwood used for making joinery. Design and build contract: A building contract in which the builder is also responsible for all or some of the design. Design check: Evaluation of the design to determine whether it conforms with the design brief and can be expected to provide a safe engineered solution. Development: The improvement of land in order to make use of it, e.g. by building structures on it or by adapting existing structures. Development can either be for the developer's own use, or else speculative, i.e. for profit. Digger: Excavators with hydraulic transmission are ubiquitous in groundwork. The first to be produced were made the J C Bamford company. District Surveyor: Borough officers first appointed after the Great Fire of London to supervise the Building Regulations. Now combined with the Building Control Officer. Doric Order: The simplest of the ancient Greek orders of architecture. The columns consist of a plain fluted shaft and a simple capital; there may be no base or a simple round one. According to Roman writer Vitruvius the order originated with a temple to Juno built by one Dorus. Dormer: A window projecting from the slope of a roof. Dowel: (Concrete) A steel bar for transferring load across a joint. (Joinery) A timber moulding with a circular cross section. Dragon beam: In traditional pitched roof construction, a diagonal tying timber across the corner of a hip. Drypack: A strong mixture of cement and sand damped with a small amount of water, used to fill holes in existing walls for example in underpinning. Ductwork: Air-handling pipes fabricated from sheet steel. Dumpy level: Originally a simple but accurate optical instrument invented in 1832 by English civil engineer William Gravatt. Now applied to any optical levelling instrument used by builders. E: Effective length: A concept used in the design of structural members. May be more or less than the actual length to compensate for the degree of restraint of the ends of the member, a member which is more rigidly held at the ends being stronger. Engineer: In English, the term is associated with engines, although this is a historical accident, the first engineers having been military engineers who were responsible for 'engines of war' such as tunnels and seige towers. Engineers engaged on public works such as canals, highways and railways called themselves 'civil engineers' to distinguish themselves from military engineers. There are now many kinds of engineer. The word itself is unprotected in the UK, so that anybody can call themselves an engineer, such as in the joke job descriptions 'rodent control engineer' and 'domestic engineer'. In France the equivalent word 'ingenieur' seems closer to words signifying ingenuity, and is a controlled designation requiring its holder to have appropriate qualifications. Engineering brick: A type of brick which is particularly strong and impermeable. The traditional product was blue in colour; other colours and qualities are available. English Bond: Alternate rows of bricks consist of all headers and all stretchers. Traditionally considered to be the strongest bond, it is often found in engineering works like bridges and retaining-walls. English Garden Wall Bond: Most brickwork bonds are designed so that one side of the wall can be built 'fair-faced' (suitable for viewing as finished work); the other side, inside the building, will be plastered so the brickwork can be left rough. Garden walls however will be seen from both sides, so Garden Wall bond is designed with a minimum number of headers so that both sides can be built fair-faced. External works: The landscaping, roads and paths created in the parts of the site not occupied by the building. Extrados: The upper surface of an arch. F: Falsework: Temporary structure used to support a permanent structure while it is not self-supporting. Fascia: In roof construction, a decorative board fixed to the ends of the rafters. Also the name board over a shop-front. Feather-edge board: A board which is thicker one side than the other. Used for fencing, where they are fixed vertically and overlapping. Sometimes found in tiled roofs, fixed horizontally, with the thicker edge at the top to provide a hanging point for tiles. Filler Joist Floor: An obsolete but commonly-found form of floor comprising a concrete slab reinforced with steel I-beams known as rolled steel joists. Fine aggregate: Sand used in making concrete, mortar etc. Firring: A piece of timber cut as a wedge and fixed to the top of a joist. Used to give flat roofs a fall for drainage, or to level up uneven floors. First fix: Electrical and plumbing first fix are the fixing of the wires and pipes in the fabric of the building, before plastering. Carpentry first fix is the provision of joists, studs and rafters. Flange: The top and bottom plates of an I- or H-beam, or of a channel. The top and bottom flanges of a beam are usually in compression and tension respectively. Flashing: Lead (or other durable metal sheets) to protect junctions of roofs and walls from water ingress. (Fr. bande (f) de recouvrement). Flat roof: A roof with a slope or pitch less than ten degrees from the horizontal. Flemish Bond: The most common bond in brickwork 225mm or more in thickness, it consists of alternating headers and stretchers, with each header being in the middle of the stretchers above and below. Fletton: The common type of machine-made yellow/orange frogged brick used in the south-east of England and London. Named after Fletton, near Peterborough. Flint-lime brick: A kind of calcium silicate brick. Flitched beam, Flitch: A timber beam strengthened with one or more steel plates bolted or screwed to it, often sandwiched between timbers. Flue: Channel formed with masonry or specially made blocks or pipes through which the products of combustion pass to the outside. Until the middle 20th century, the need to stack flues from storey to storey imposed a discipline on architecture which is now absent. Fluid Mechanics: The science of the properties and motion of liquids and gases. Flush pointing: Flush with the surface of the bricks. Foot: Unit of length in the Imperial system; one-third of a yard, equal to 304.8mm. Force: That which can accelerate a mass. An example of a force is weight, which acts to accelerate any mass towards the centre of the earth. Structural engineering is about providing structures which are strong enough to resist the weight and other forces acting on them. In the SI system, force is measured in Newtons. Foreman: Trades foremen, for example foreman plasterer, electrician, are in charge of their tradesmen on a site. The general foreman is in charge of the trades foremen. The term does not specify gender. Formwork: A mould into which concrete is cast. Foundation: The part of a building or structure which transmits loads to the soil. Foundations may be stepped masonry, mass or reinforced concrete, or piled. (Fr. fondation, f). Frame clamp or cramp: Metal component screwed to the window or door frame and built into the masonry wall. Frenchman: A tool for forming the shape of pointing. Frog: The recess in a machine-made brick. Furlong: Unit of length in the Imperial system; ten chains, or 660 feet, one-eighth of a mile, equal to 201.168 metres. G: Gable: The triangular wall at the end of a building with pitched roofs. (Fr. pignon, m). Ganger: The leader of a work gang, for example, a concrete gang. Gauge: A measuring rule. Also, the height of brickwork, specified as the number of courses per foot or per 300mm. In the south of the UK brickwork gauge is almost universally four courses per foot or per 300mm. Ginny wheel: Pulley used for hoisting things up a scaffold. Glass bead: Moulding used to retain glass in a window frame. Gram: Unit of mass in the SI system of weights. Symbol g. Grating: Iron or plastic protection over a gully. Gravel: Naturally occuring ballast or course aggregate. Green Roof: A flat roof covered (deliberately) with growing material. Groundwork: Foundations, drainage, levelling and other building operations involving digging. Grout: Cement mixed with enough water to make it runny, used to fill a gap under the base of a steel column. Also the filler between wall tiles. Guarding: Protection against people or things falling off the edge of stairs, landings, balconies or scaffolds. Gully: A container with water in it, to seal the inlet to a drain and prevent the release of noxious gases. Gutter: Open channel for receiving and carrying away rain water. (Fr. gouttière, f). H: H-section: A steel component shaped in cross-section like an H, such as a Universal Column (qv). Half timbered: A descriptive term for a traditional timber-framed house. Hammerbeam roof: A form of historical roof truss, usually comprising a central truss section spanning between two cantilevers. Handrail: A length of timber or metal at hand height at the side of a staircase or landing. Hardwood: Timber from a deciduous tree; note that hardwood can be softer than softwood, for example balsa wood is a hardwood although very weak and soft. In construction, hardwood may be used in preference to more readily-available softwood because of its higher strength, its greater durability, or its superior appearance. Efforts should always be made to ensure that the timber is from renewable sources. (Fr. bois (m) feuillu). Header: A brick whose 'head' or short end is visible on the surface of the wall. See stretcher. Hearth: Fire resisting area of floor adjacent to an open fireplace. High alumina cement (HAC): Concrete made with this type of cement hardens faster than with Portland cement. This advantage once led to HAC being used for manufacturing precast concrete elements, but it has the disadvantage that it tends to become weaker over time especially in a moist atmosphere. The collapse of some swimming pool roofs in the 1970s led to HAC being banned for structural use. It is still used for non-structural purposes, for example, for bedding sanitary ware on a concrete floor. High strength friction grip bolt: Used for connecting steel components in situations where it is not desirable for the connection to slip. High tensile steel: A grade of steel stronger than mild steel, which may be used both in structural steelwork and concrete reinforcement. Hip: A roof feature in which two pitched roofs meet at a corner; the rafter forming such a junction. The hip rafter is not usually a load bearing member. (Fr. arête (f) de croupe). Hipped roof: Featuring hips. Hod: A three sided container mounted on a pole, used to carry bricks or mortar up a ladder. Hod carrier: Bricklayer's labourer. Hoist: An elevator for lifting goods and, usually, people up a scaffold. Hollobolt: Proprietary expanding bolt which can be used in making bolted connections to hollow sections, and other situations where lack of access prevents a nut being used. Hollow section: A tubular structural steel member, either circular ('CHS'), rectangular ('RHS') or square ('SHS'). Elliptical hollow sections are also available. Honeycomb brickwork: Built with gaps between the bricks, to allow ventilation. Hundredweight: In the UK imperial units system, a weight of 112 pounds, also equivalent to eight stone, or one twentieth of a ton. Hydration: The process by which cement hardens by reacting with water. Hydraulic cement: Cement which sets under water, like Portland cement. I: I-section: A structural steel section shaped like an I, such as a Universal Beam. Imperial system: The traditional system of weights and measures used in English-speaking countries until superseded by SI units in a process often called metrication, which took place in the UK in the early 1970s. The principal Imperial elements are yards (with their subdivisions of feet and inches), and pounds (divided into ounces and multiplied into hundredweights and tons). Imposed load: The weight of furniture, people, storage, and any other non-permanent loads. Inch: Unit of length in the Imperial system; one-twelfth of a foot, equal to 25.4mm. Intrados: The underside of an arch. Ionic Order: One of the ancient Greek orders of architecture, characterised by a fluted column and a capital consisting of four volute scrolls. Named after Ionia in Greece, where it was first used. Iron: An element, which is one of the most common on earth, and the principal component of steel. J: Jack rafter: A rafter that is shorter than a common rafter because it is intersected by a hip or a valley. Jetty: In traditional timber-framed buildings, the projection of an upper storey over the storey below. The reason for this form of construction seems originally to have been simply to increase the floor area of the upper storeys. Jiffy hanger: A proprietary steel component which enables a joist to be connected to another timber running at right angles. Joinery: The fabricated timber components of a building such as doors, windows and staircases. (Fr. menuiserie, f). Jointing: The process of finishing the mortar between bricks or other masonry units at the time of building, as opposed to pointing the joint later. Joist: (Timber) Horizontal member which is one of a group running parallel and close together, supporting a floor or flat roof. (Fr. solive, f). Joist hanger: Proprietary steel component to support the end of a joist so that it does not have to be built into the wall. K: Kentledge: Heavy weights used to counter balance a load or provide a reaction. Keystone: The centre stone of an arch, if it is larger than the ordinary voussoirs. Kicker: In reinforced concrete construction, a concrete plug typically 50 to 100mm high to help locate the formwork for a wall or column. Kilogram: The principle unit of mass in the SI system of weights and measures. Equal to 1000 grams. Abbreviation kg. Approximately equivalent to 2.2046 pounds. KiloNewton: One thousand Newtons – the unit of force in the SI system. Newtons are very small, and the kiloNewton is the practical unit most often used by engineers. In imperial terms it is approximately equivalent to the weight of two hundredweights. Abbreviation kN. King post truss: Roof truss with a central vertical member. L: Labourer: General labourer: building worker without any specific skill. Specific trades have their own labourers such as bricklayer's, plasterer's, labourer, whose job is to set up scaffolds and carry materials. Lacing: Generally horizontal members that connnect together and reduce the unsupported length of compression members. Laminated strand lumber (LSL): A type of reconstituted timber made of seperated strands glued together under pressure. Lath: A thin strip of wood nailed to studs or joists as a carrier for plaster. Early laths were riven (split with a blade); in more modern times they were sawn. Expanded metal lathing (e.m.l.) is used for the same purpose, especially for external work with sand and cement render; internally, laths have been superseded by plasterboard. Ledger: In scaffolding, the horizontal members running along the scaffold. They support transomes or putlogs. Level: Horizontal; the instrument used for checking whether things are horizontal. Levels on a drawing are heights above a recognised datum which might either be the Ordnance Datum or a local datum for the job, whose location and value has to be specified on the drawings. Levelling: Finding levels during surveying, or providing levels for new construction. Lewis: A device consisting of expanding wedges used for lifting heavy stone masonry. Lift pit: Every lift has (by law) to have a clear space below its lowest landing level, fitted with equipment intended to bring to a safe halt a lift which has failed to stop. This often requires a lift pit, typically 1.2 to 1.5m deep. Lime Mortar: The traditional form of mortar, it is soft and flexible and liable to dissolve slowly in rain water. It is still available for use in restoration work. Lintel, lintol: A short beam over a door or window opening; may be steel, concrete or, traditionally, timber. The spelling with an 'o' is traditionally favoured by draughtsmen; the 'e' however is given authority by the King James bible (Exodus 12:22 etc). (Fr. linteau, m). Live load: Imposed load. Load bearing: Designed to support a load in addition to its own weight. Load factor: Engineers design structures to support loads which are more than the maximum load expected. The actual loads are calculated as accurately as possible and then multiplied by the factor. Typical load factors are 1.4 for dead loads and 1.6 for imposed loads. London stock: The stock bricks made in the London area for centuries. M: Manhole: Hole in the ground to allow access to underground services; access chamber. Mansard: A roof which slopes steeply (e.g. 15 degrees from the vertical) to allow more space inside the roofspace. Named after a French architect. Masonry: In general usage this describes work constructed of stone, but technically the term masonry also includes brickwork and blockwork. (Fr. maçonnerie, m). Mass: A property of all matter. It is measured in, for example, grams. Mass is independent of gravity, unlike weight which depends on gravity. Mass concrete: Unreinforced concrete, as often used in foundations or other applications where the added strength of reinforcement is not required. Maul: Large wooden hammer used in masonry and paving work. Method statement: A document which shows how the construction will be carried out safely. Under most forms of contract the Contractor will prepare any necessary method statements and the Engineer will usually check them. Method statements are also sometimes required by neighbouring owners where potentially hazardous work is being proposed, or by Planning authorities to ensure that a proposal is buildable. Metre: The basic unit of length in the SI system of weights and measures. Multiplied and divided by 1000 to give derived units such as millimetres and kilometres. Symbol m. In the USA the spelling meter is used. A metre is approximately equivalent to 3ft, 33/8 inches. Metric: The UK construction and engineering industries were encouraged by the Government to adopt the metric system in the early 1970s. The system used was and remains (rather shortsightedly) based on millimetres rather than the centimetre system taught in schools in the UK and around the world. See SI system. Mews: In London and other large cities, the stables belonging to large houses would often be accessed from a small road running along the backs of the properties, known as the mews. The mews properties are often separated from their main house and converted to sought-after dwellings. Mild steel: Structural steelwork and reinforcement generally come in two qualities: mild steel and high-tensile steel, the latter being stronger but more expensive. Mile: Unit of length in the Imperial system; 1760 yards, equal to 1609.344 metres. Mix: The proportions of the ingredients of concrete, mortar and such like. Mobile crane: Versatile lifting devices in a range of sizes, usually telescopic. Mock Tudor: An architectural style popular in suburban development in the twenties and thirties, in which traditional styles were copied poorly. Modulus of elasticity: A measure of the amount by which something can be deformed by a force and recover when the force is removed. Moment: Short for bending moment. The bending force which acts on, for example, a beam, and is resisted by an equal internal resistance moment within the beam. Mortar: A binder for masonry. The traditional product was Lime Mortar; modern mortars rely upon cement mixed with sand, with the addition of lime or plasticizer added to make them workable or 'buttery'. (Fr. mortier, m). Mortice and tenon joint: A traditional way of joining two timbers at right angles: the one coming in from the side is reduced to a tenon, which fits into the cavity or mortice and is secured by glue or nailing. Moulding: Timber (or other material in imitation of timber) shaped into a pattern and used for decorative details such as skirting, picture rail and so on. Traditional moulding shapes include quadrant, ogee, torus, chamfered, glass bead, half round, dowel and so on. Moulding pin: A very thin pin or nail used for securing mouldings. N: Nail: Metal spike used as a fixing for timber. Needle: As a noun, a short beam introduced through a wall to provide temporary support while the wall is being re-supported. As a verb, to insert such beams. Neutral axis: The point near the middle of a beam's cross-section which experiences neither tension nor compression when the beam is subjected to bending. Newlyn datum: See Ordnance datum. Newton: The principal SI unit of force. It can be thought of as equivalent to the weight of Sir Isaac Newton's apple. Node: Theoretical point where two or more members are considered to be connected together. Noggin (or nogging, naggin etc): A short length of timber fixed crossways between joists, studs or rafters; also the infill between the studs of a traditional timber-framed building. O: Ordnance datum (OD): The national leveling standard for the UK, the basis for levelling set up by the Ordnance Survey, representing mean sea level at Newlyn, S Wales. Ordnance Survey: The organisation which makes and maintains accurate maps of the United Kingdom. The maps were originally for military purposes but are now used for land use planning and development of land. Oriel window: A bay window that projects from the wall and does not have its own foundations. P: Padstone: A block of concrete or stone used to spread the weight of a beam or joist, to avoid crushing the wall upon which it rests. Parallel flange channel: A form of steel channel. Pargetting or pargeting: (pronounced pargeing). Rendering, especially (1) decorative sculptured rendering on the outside of a building, found mainly in East Anglia (2) the render (traditionally consisting of cow manure) lining the inside of a flue, formed into a cylindrical tube by pulling up an iron sphere on a chain. Partition: A non load bearing wall between rooms or areas in a building. Partitions may be of any material but are often studwork. Party wall: A wall shared between two buildings. Laws have existed for many years, particularly in London but now throughout the UK, for governing the building, alteration and maintenance of party walls. (Fr. mur (m) mitoyen). Pea shingle: Shingle consisting of rounded stones that pass through a 10mm grid. Permissible stress: Stress that can be sustained safely. Codes of Practice for structural design used to specify permissible stresses with which the actual stress was to be compared. Perp.: The vertical mortar joint between two bricks (bricklayers' slang). Picture frame: In structural engineering, a rectangular steel frame consisting of two columns and two beams, sometimes used when a load bearing wall has to be removed. Pier: A masonry column; a jetty. Pile: A foundation consisting of a deep column extending down into the ground, used when the foundation needs to get support from a deeper and stronger or more stable layer. Originally piles were timber (often elm) but they can now be concrete or steel or even aluminium. Bored piles are made by pouring concrete into a hole drilled in the ground whereas driven piles are ready-made and driven into the ground. There are many ingenious proprietary piling systems and piling can be used both for new buildings and for strengthening or stabilising the foundations of existing buildings. Contiguous piles are used to form a retaining-wall. Pile cap: A (normally reinforced concrete) structure transferring loads from the building into the piles. Pile driver: Machine for hammering or forcing piles into the ground. Piling rig: A machine which drills a hole in the ground for a cast-in-situ pile. Pitch: Of roofs, the angle of the rafters from the horizontal. Traditionally the pitch was expressed as the number of vertical inches corresponding to twelve horizontal inches, thus a 45 degree roof was described as a twelve inch pitch. Pitched roof: One whose slope exceeds ten degrees. Planning: The legal system, operated and enforced by local authorities, by means of which the development of land is controlled for the public good. Not to be confused with Building Control. Plaster: The material which is spread to leave a smooth surface on a wall or ceiling. The main binding material may be cement (when it is known in the UK as render), or lime, or gypsum, the latter two being restricted to internal use; in any case there will be a filler of sand, or in proprietary prepackaged plasters, powdered vermiculite. (Fr. plâtre, m; enduit (m) interieur). Plasterboard: A sandwich made of two sheets of cardboard with a gypsum plaster filling, typically 9mm or 12mm thick. Nailed or screwed to studs, joists or rafters as a carrier for a plaster skim finish, or plasterboards with chamfered edges can be jointed so that they act as a finish without being skimmed with plaster. Plasterboard helps to provide the resistance to fire of buildings. (Fr. placoplâtre, m, from a trade name). Plum: A large stone or piece of solid concrete used as a filler in mass concrete. Plumb: Vertical or verticality, measured using a plumb-line or plumb-rule or these days a spirit level. Pointing: The surface treatment of the mortar between bricks or other masonry units. There are various styles of pointing: flush, struck and weathered, recessed, tuck, bucket handle etc. (Fr. jontoiement, m). Poling board: A short strong board used in the temporary timbering of excavations and tunnels. Portal frame: A structural frame consisting of two columns and a cross- beam, with rigid connections. Often used for single-storey warehouses and workshops. The cross-beam is often formed as two rafters to make a pitched roof shape. Portland cement: A hydraulic cement used almost universally for making concrete and other cement based products. So-called because concrete made with it resembles limestone from the Isle of Portland. Post stressed concrete: Concrete strengthened with steel wires which are stressed after the concrete has cured. Pound: The unit of mass in the imperial system of weights and measures. Confusingly, the same word is also used sometimes for a unit of force, more accurately called a pound-force. The UK's unit of currency called a pound was originally the value of a pound of 'sterling' silver. Poundal: A unit of force in the imperial system of weights and measures. Precast concrete: Concrete components made in a factory or yard and transported to the site. Prestressed concrete: Concrete strengthened with steel wires which are stressed before the concrete is poured. Professional indemnity: Insurance against claims against a professional person or practice. Progressive collapse: The process wherein the collapse of part of a building leads to the collapse of an adjacent part in 'house of cards' fashion. Pugging: Traditional infill between timber floor joists intended to enhance the acoustic insulation of the floor. It may occupy the whole depth of the floor or only part of it. Materials used include sand, mortar, concrete, straw and sea shells. Pulverised fuel ash: A fine white powder resulting from burning powdered coal in power stations, which can be used to supplement cement in making concrete for civil engineering works. Purlin: A horizontal structural member which supports a sloping roof covering, with or without rafters, and which carries the roof loads to the primary framing members. (Fr., panne, f). Putlog or putlock: A horizontal scaffold member one end of which is built into the wall. Putlog scaffolds are not much used these days because they can be dangerous, and because the hole in the wall has to be repaired when the scaffold is taken down. (Fr. boulin, m). Q: Quadrant: A quarter of a circle. The name is also used for various things in this shape, such as a timber moulding, a corner kerbstone, or a historic navigational instrument. Queen post truss: A truss with two posts directly supporting the purlins. Quoin: The external corner where two brick walls meet. R: Rafter: Sloping structural member supporting a roof. (Fr. chevron, m). Ready-mixed concrete: Mixed in a batching plant and delivered in ready-mix trucks. Recessed pointing: Flat pointing set back from the surface of the bricks. Rectangular hollow section: A structural steel component in the shape of a steel tube with a rectangular cross section. Reinforced concrete: Concrete reinforced with steel bars to make a versatile structural material which is very strong in bending, shear, tension and compression, unlike plain concrete which is strong only in compression. Reinforcement: (Also known as rebar). Steel bars for reinforcing concrete. They are bent into special shapes according to the Engineer's bending schedule, and fitted into the correct position by a skilled operative called a steelfixer. Render: Cement-based wall plaster. Retaining wall: Retains soil on one side. May be made of masonry, reinforced concrete, or various other traditional or proprietary structural systems. Retention: A percentage withheld from a contractor's payment until an agreed time after the work is complete. Ridge: The top of a pitched roof, where roof planes that slope in opposite directions meet. (Fr. faîte, m). Ridge board: A thin timber used to align the tops of the rafters. In most roofs the ridge board is not a load bearing member. (Fr. planche (f) faîtière). Ridge tile: A curved tile which covers the ridge on a pitched roof. Riser: Vertical board rising from the back of one tread of a staircase to the front of the next. Rising damp: Water soaking up through the walls of the building. May be prevented by the use of a damp proof course in the walls. Rivet: Before structural steel I and H sections became available engineers made up sections by joining narrow plates together using steel rivets with a head formed by hammering while red-hot. Rivets are no longer used for connecting structural steelwork in the UK, with fabrications mostly replaced by ready made sections, and with bolting and welding available which are both faster and safer for connections. The presence of rivets in an existing structure can help in dating it, and usually indicates steelwork dating to before about the 1950s. Rolled steel joist (RSJ): One of a range of I- and H-shaped steel members. Only small sizes of joist are still produced, most of the larger sizes having been replaced by Universal Beam and Universal Column sections. RSJs were originally devised for use in filler-joist construction. Rough arch: A brick arch in which the bricks are rectangular and the arch shape is formed by means of the mortar joints being wedge-shaped. (cf 'axed arch'). S: Sand: Aggregate consisting of mineral particles whose size is generally less than 5mm; fine aggregate. Merchants in the UK supply soft sand and coarse or fine sharp sand. Sand-lime brick: A kind of calcium silicate brick. Sash window: The traditional type of window which opens by sliding up and down. The frame is called a box-frame, because the side members are hollow wooden boxes inside which the counterweights slide up and down. The biggest problems with them are that over-zealous painting leaves them jammed shut, and the sash-cords have frequently to be replaced. Modern versions are available incorporating draught proofing and springs instead of weights. Scaffold: A framework for temporary access to building works. The traditional way to build a scaffold in the UK used to be with timber poles connected together with wire bonds. Standardised 1 15/16 inch (49mm) steel tube with proprietary steel connectors came into widespread use after the second world war, based on war surplus tubing that had been used in beach defenses. Various proprietary scaffolding systems are also available and may cost less, but "tube and fittings" scaffolding has the advantage of flexibility. (Fr. échafaudage, m). Scaffold board: Timber boards used to make walkways on a scaffold. Scantling: The cross-sectional dimensions of a length of timber; the principal dimensions of a shaped stone; a piece of timber of a specific size. Scarf: A traditional woodworking joint for extending the length of a timber. Screed: A temporary rail, installed at a specific level, to enable concrete to be finished at the correct level. Also sand and cement, mixed rather dry, laid on a (usually concrete) floor and screeded and trowelled to make a smooth surface. (Fr. chape, f). Screw: Threaded fastener. Secant piles: Contiguous piles where each pile cuts into the one before, to make a more-or-less waterproof retaining-wall. Second fix: (See first fix). Work which takes place after plastering, for example, fixing light switches, skirtings. Services: See: Building services. Setting-out: The process of making sure that a building or structure is built in the correct position and the right size. Settlement: The small downwards movement of foundations when the weight of the building comes onto them, due to compression of the soil. Tends to be negligible in clay soil but can be significant in sand. (Fr. tassement, m). Shake: A defect of timber: damage caused by rough handling. Sharp sand: Sand which, unlike soft sand, does not include fine silt or clay particles, making it more suitable for use in concrete and screed. Shear or shear force: The force which tends to make the top and bottom flanges or fibres of a beam move parallel to one another. The web of the beam resists the shear force, which is at its greatest at the ends of the beam next to where it rests on its supports. Sheerlegs: A lifting device using two timber poles fixed together at the top. Shingle: Aggregate consisting of stones whose size is between 5 and 10mm. Also, a wooden roof tile. Shuttering: Formwork. Sill: Projecting moulding at the bottom of a window or door. (Also spelled cill). Simply supported: Describes a beam which rests on a support at each end, that is, it is not supported at more than two points, is not held rigidly by the supports, and does not form part of a larger framework. Skirting: Timber or other moulding around the base of a wall. Sleeper wall: Supports a timber ground floor, and is often built in honeycomb brickwork to allow ventilation of the space under the floor. Soaker: A metal sheet bent at a right-angle, part of the waterproof flashing of the junction of a tiled or slated roof abutting a wall. Soffite: The underside of a building component such as a lintel or beam. A board fitted to the underside of the ends of rafters or flat roof joists. Soft sand: Sand which includes fine silt or clay particles, which make it more suitable for making mortar or render than sharp sand. Softwood: Timber from a coniferous tree, i.e. most of the timber used in construction. Softwood timber comes in a variety of grades, the most common for structural use being classes C16 (for general use) and C24 (stronger timber with fewer knots and defects). (Fr. bois (m) resineux). Soil: In engineering, the soil is all the solid materials below the earth's surface, including rock, sand, clay and so on. Soil Mechanics: The science of the strength of soil. Soldier: A vertical member in a retaining-wall, especially in temporary works. Sole plate: A timber placed on the floor as the base for a partition. Special (brick): A brick specially made in a non-standard shape. Special foundations: Defined, in the Party Wall act, as foundations incorporating steel. Spine wall or partition: In traditional domestic construction, a load bearing partition between the front and rear rooms of the house. It supports the upper floors and, usually, the roof. Splice: A steelwork connection for joining (for example) two lengths of column to form a longer column. Beams can also be spliced, but the splice must not, if possible, be in the middle of the beam where the bending moment is greatest. Springing: The masonry supporting an arch. Square: Rectangular, or at a right angle; the tool used for checking rectangularity. Square hollow section: A structural steel section in the shape of a square tube. Squint: A special brick for use on a corner which is not a right-angle. Stanchion: Steel column. Standard: A vertical tube in scaffolding. Steel: A metal based on iron, with the addition of carefully defined quantities of carbon and other elements to produce a metal with specific qualities. Structural steel is used for steel frames and is weldable and easily cut and shaped. Steel reinforcement (qv) is designed to be cut and bent to shape. Modern steel use dates from the invention of the Bessemer converter, and the modern product differs from the older types of steel from which weapons were made. (Fr. acier, m) Steel angle: A structural steel component, the cross section of which is L-shaped. Steelfixer: A worker who specialises in placing reinforcement for reinforced concrete. Stepped flashing: Metal flashing cut in a stepped pattern to waterproof the junction of a tiled or slated roof with a brick wall. Stock brick: The traditional handmade brick without a frog, made by moulding clay in a wooden mould or 'stock'. Strain: The amount by which something has changed length, measured as a percentage of its original length. Strap: A component, usually steel, installed to ensure that walls are connected to and restrained by floors. Stress: Force divided by area, measured in (for example) Newtons per square millimetre, or pounds per square foot. Stress graded: (Of timber) tested and marked with a strength grade. The two grades of softwood most used in construction are C16 or General Structural grade, and C24 or Special Structural grade. Stretcher: A brick whose longest side is visible on the surface of the wall. See header. Stretcher bond: A brickwork bond consisting only of stretchers, suitable for half-brick thick walls and cavity walls. Stringer: Angled structural beam supporting the treads and risers of a staircase. Strike: Dismantle (scaffold or falsework). Struck and weathered pointing: Finished with a sloping surface, recessed slightly at the top and protruding slightly at the bottom of the joint. Structural Engineering: A branch of engineering dealing with structures, such as buildings and bridges. In the UK structural engineers became distinguished from Civil Engineers when they started to specialise in the new structural material reinforced concrete in the early 20th century, although they soon began to work in all structural materials. Structural glass: Glass used in situations where it will or may support more than just its own weight. Glass balustrades, stairs and floor panels are becoming common. Structural steelwork: A frame of steel sections supporting other parts of the structure. Stucco: Rendering shaped and painted to resemble ashlar stonework. Stud: A timber post in a studwork partition or in traditional timber-framed construction. There are also steel studs made of lightweight galvanized steel. Studwork: A type of partition formed from studs at close intervals, traditionally clad with lath and plaster, now with plasterboard. Subsidence: A downwards movement, especially a movement of foundations. The term is most often used to describe the movement of foundations on clay soil, when the soil shrinks due to becoming drier. (Fr. affaissement, m). Sulphate/sulfate: Sulphates in soil or ground water can damage cement- based blocks, mortar or concrete. Special sulphate-resisting cement can be used to resist it. Sulphates in the ground are often a result of industrial pollution. Systeme international (S.I.): The system of units, based on the metre, kilogram and second, used by engineers in the UK and elsewhere. The metre and kilogram are divided and multiplied by 1000 to make larger and smaller units. Many think it is an odd system which is based on a unit, the kilogram, which is itself a multiple of another unit, being 1000 grams. T: Temporary bench mark: A levelling base point of known level. See bench mark. Temporary works: Propping or shoring to enable the permanent works to be carried out. Tension: A pulling force, such as that experienced by a cable, or in the bottom flange of a beam with a load on it. Theodolite: An optical instrument used by land surveyors for surveying and by engineers and builders for setting-out lines and angles on the ground. Tie: Any member which provides a tensile force to tie two other members together, especially, the bottom horizontal member of a roof truss, and (in a steel framed structure) steel beams whose main function is to tie columns together. Tile: Ceramic unit for wall decoration or roof weathering. Timber: Wood suitable for use in construction. In the UK it is usually softwood. (Fr bois, m). Timber connector: Various kinds of steel fixings designed to make high-strength connections in timber construction. Timber-framed: Construction in which the main load bearing elements are timber. Traditional timber-framed or 'half-timbered' houses are one example; modern timber framing uses timber load bearing panels made of studwork clad with plywood. Ton: Unit of mass or weight in the imperial system of weights. The UK or 'long' ton is equal to 20 hundredweights, 2240 pounds, or 1016 kg. In the US a 'short ton' of 2000 pounds is used. Tonne: Unit of mass in the SI system. Equal to 1000 kilograms. Top plate: A horizontal timber on top of a partition to receive the floor or roof timbers. Tower crane: A crane with the jib mounted at the top of a tower, to give clearance over obstructions. They may be static or tracked, with a rigid or 'luffing' (vertically hinged) jib. They are usually electrically operated. Town planning or town and country planning: The original name of the discipline and process which is these days generally known simply as planning. Trade: The various types of construction workers: electricians, carpenters, joiners and such like. Tread: A single step of a staircase. Transome: A component of scaffolding: a horizontal tube supporting the boards. Also a horizontal member in joinery, for example the part of the frame between an upper and lower window. Tree preservation order: An order under planning regulations, protecting a tree or group of trees from damage. Trimmer: A joist which carries extra loads, for example, those due to an opening or a partition. Trimmers should be stronger than the normal joists. Traditionally they were thicker, these days extra strength is achieved by bolting two or more timbers together. Truss: An arrangement of steel or timber components designed to span across a large distance to support a roof, floor or bridge. Trussed rafters: Wooden trusses, usually triangular in shape, spanning between the external walls at 600mm centres or thereabouts to form a roof. They are cheap and easy to use for new roofs and do not require internal support from beams or partitions, but their disadvantage is that they restrict the use of the loft space more than conventional 'cut timber' roofs. Tuck pointing: A difficult and expensive form of pointing. The joint is flush pointed with mortar coloured to match the bricks, and a very thin false joint is cut into the mortar and pointed in lime putty of a contrasting colour. Very difficult to get done today – the art is nearly lost. Tuscan order: The plainest of the five classical orders of architecture, similar to the Doric but with a plain rather than fluted shaft. Tusk tenon joint: Traditional timber connection, typically used to connect trimmers around a hearth. The tenon extends through the main joist and is fitted with a wooden wedge to stop the joint from opening up. In modern construction a steel bracket would be used instead, unless one were restoring a historical building. U: Underpinning: Making existing foundations deeper (by extending them downwards). Usually done with mass concrete but other high- and low-tech methods are available. Universal Beam: A standardised steel component which is I-shaped in cross section. Over 70 different sizes are available in two main steel grades. The Universal Beam and Universal Column were introduced in the late 1950s and were based on American patterns, and rolled in new 'universal' rolling mills. They replaced a range of sections which had been developed by various UK manufacturers over the preceding century. Universal Column: A standardised steel component which is H-shaped in cross-section. About 30 different sizes are available in the UK, in two main steel grades. The same comments apply as to Universal Beam above. V: Valley: The meeting of two roof planes at an internal angle; the rafter which forms the junction. Valuation: Building work is valued monthly by the Quantity Surveyor or Contract Administrator. Vanity unit: Washbasin built in to the top of a cupboard. Variation: A change to the building contract due to an instruction issued by the Contract Administrator. Vault: An ancient form of construction consisting of masonry formed in an arched shape. Vermiculated: Of stonework: carved in a random pattern fancifully comparable with the appearance of worms. Vermiculite: An expanded mineral used as lightweight aggregate in concrete and other filling applications. Vierendeel girder: A type of truss consisting of vertical and horizontal members arranged like a ladder on its side. Voussoir: One of the stones or bricks forming an arch. W: Waling: Horizontal steel or timber member in a retaining-wall, especially in temporary works. Wane: A defect of timber. The timber section is too small because it was cut too close to the edge of the trunk. Web: The middle plate of an I-beam, H-beam or channel. The web connects the two flanges, and resists shear forces. Weight: A force resulting from the effect of gravity on a mass. Welding: A technique for joining steel components by the deposition of small drops of molten steel which bonds to the parent metal. Wind load: Engineers have made great efforts to understand wind loading since the Tay Bridge disaster in 1879. Withes: (Pronounced whiffs)The usually half-brick thick dividers between flues in a chimney. Woodscrew: Threaded fastener for use in wood. Y: Yard: The principal unit of length in the Imperial system; three feet, equal to 914.4mm. Young's modulus: A measure of the elasticity of a material. Defined as stress divided by strain; see modulus of elasticity.