Glossary of Building and Engineering Terms
Note: This is an informal glossary of terms used in domestic architecture and structural engineering – it is neither comprehensive nor authoritative, but we hope it will be helpful and possibly interesting!
Acrow Prop The ubiquitous tubular steel prop used for temporary support in building projects everywhere. The height of the prop is adjusted by a screwed collar between the upper and lower telescopic tubes, via a short handle. The working capacity of a typical 3 metre prop is 10kN (1 tonne), but eccentric or inclined loading can significantly reduce this. Care must be taken to arrange and use props - it is quite possible to over-tighten the prop and lift up the wall or floor you are trying to support, causing unnecessary damage. Strongboy heads are a useful accessory that enable the prop to be offset from the supported wall - however the working capacity is only 340kg which is not enough to support a cavity wall or solid brick wall at the normal spacing!
Action A term for force, adopted for the Eurocodes. This implies movement but in the case of a structure is generally intended to remain static. Has a historical context in Newton's third law: "To every action there is an equal and opposite reaction".
Aggregate in construction, used to describe granular materials such as gravel and sand. In concrete, fine aggregate (sand) and coarse aggregate (gravel) are essential constituents together with cement.
Architecture One of the fine arts (apparently); the art and science of building. Sometimes, there's more art than science, from an engineer's point of view.
Basement A storey of a building below the principal storey, usually below ground level. A cellar is defined as any underground room or vault, usually taken to mean for storage use rather than habitation. When considering extending a building below ground, waterproofing and resistance to earth pressure are key design issues. Party Wall agreements and the support of nearby structures are usually significant.
Beam A structural member designed to resist bending; usually horizontal. Beams may also be inclined, or cranked (with a bend in the vertical plane) to suit the shape of a roof, for example.
Bearing The point of support for a beam, such as on a wall. The concentrated load on the bearing must be distributed into the supporting wall, and reinforced by a padstone if necessary. Bearing length is the length of beam resting on the wall; generally 150mm, but this may be varied by the engineer to suit requirements.
Buckling A mode of failure caused by instability in a structural member, as increasing load leads to distortion out of the plane of the applied force. This can happen to columns and also beams that are not laterally restrained.
Cement The constituent of concrete or mortar that binds the aggregate together to form a solid material. Portland cement is mainly composed of calcium silicates and formed by sintering limestone and other minerals in a kiln, before grinding to powder. The composition depends on the chemistry of the locally available raw materials, but is tightly controlled to give uniform performance.
Column A vertical member designed to resist axial load (direct compression), such as the weight of a building. May be made of any suitable material such as concrete, masonry, steel or timber. Design is governed by the slenderness (ratio of length to breadth) to resist the tendency to fail by buckling.
Compression An axial force in the "pushing" or "squashing" sense; the opposite of tension. Columns are designed to resist compression, but it is also produced in bending of beams.
Concrete An economic and widely available material comprising aggregates and cement. The design of the mix can be varied to produce the required strength, workability and resistance to chemical attack. Water is required for the chemical reaction of curing concrete, which is quite different from "drying out".
Dead Load The force on a structure resulting from the weight of the structure itself. This is normally constant, except of course during construction or demolition when the changes in load need to be considered to ensure safety at each stage.
Deflection The change in shape resulting from the application of a force or load. Beams deflect due to bending, even when made of steel or concrete. Limiting deflection to an appropriate value is often the governing consideration in design of buildings, to ensure serviceability in use. Bouncy floors, sticking doors, leaky roofs and cracked finishes are symptoms of excessive deflection.
E (Young's modulus) See Young's modulus.
Engineer A contentious issue, this one! A person who designs or makes or puts to use engines, machines, structures or public works such as canals, bridges and harbours etc. Some of them grumble about the term being used for mechanics who fix washing machines or the folk who unblock drains, however most engineers ought to help out if you ask politely. Despite some of the world's most famous engineers having come from Britain, the term is not protected and there is no restriction on who can practice as an engineer, whereas in other countries the engineering profession is allegedly held in the same esteem as the medical or legal profession. Most good engineers enjoy their work too much to worry about their social standing!
Engineered Timber The product of manufacturing techniques use to improve the properties of natural timber, such as increasing its strength or reducing its variability. Glulam and I-joists are common examples, as is plywood or OSB. In the US, timber used for construction is called Lumber.
Engineering Brick Special strong bricks used for high-strength applications, such as beam bearings or civil structures. Class A (often blue in colour) and class B (red) have design crushing strengths of 70 and 50 N/mm2 respectively, compared to 10 for stock bricks and 20 to 28 for common or facing bricks. They are also more resistant to water absorption and used for damp-proof courses or underground structures.
Eurocodes The system of standards for structural design developed over 35 years and finally introduced in 2010. These are intended to provide a common basis for design across the EU and replace the familiar British Standards. Many of the principles are common to the British Standards but the notation and terminology are quite different, so engineers need to be familiar with both methods at this time.
Factor of Safety A figure by which the design strength of a structure is reduced (or the design load increased) in order to ensure the structure has a reserve of strength against failure or collapse. This is intended to cater for variations in material strength or uncertainties in determining the design load, but not for laziness in design or shoddy construction! The factors used in practice are lower than the lay person might expect – 35% to 60% for dead and imposed loads, but up to 250% for materials in masonry structures.
Force Fundamental to engineering science, an influence that causes a body to change its speed, direction or shape. Structures are usually intended to resist movement, so the effect of the force is to change the shape of the structure, causing deflection. The stiffness of the structure determines the amount of deflection for a given force.
Foundation The part of a structure that transfers the forces in the building into the ground. Being invisible, the importance of the foundation is often overlooked, but foundation failure is the most frequent and expensive cause of warranty claims. Inevitably, the strength of the soil supporting the foundations is critical to their performance and a good understanding of soil conditions is vital.
Glulam Is the term for glued laminated timber, by which beams and truss members can be built up of smaller timber plies to form large sections with consistent properties, in factory conditions. Glulam beams are manufactured in standard sizes but can be custom-made to special shapes, or supplied pre-cambered to allow the dead-load deflection to be cancelled out when installed.
Heave Ground movement causing upward movement. Commonly caused by the tendency of clay soils to swell when absorbing water, which is a frequent cause of damage to buildings. Heave precautions in foundation design include compressible void formers that are designed to collapse under the swelling pressure without transferring that pressure to the building. Removal of mature trees from clay soils can cause heave as the moisture content in the soil recovers from the deficit accumulated by successive seasons of water abstraction during growth. Frost heave is caused by the formation of ice within granular soils and is critical for cold store buildings. Other causes of heave include the chemical reaction between concrete floor slabs and inappropriate materials in the hardcore sub-base, such as gypsum plaster.
High-Strength Friction Grip (HSFG) Bolts These are a special type of bolt designed to clamp steel plates together with a force sufficient to prevent slippage under service loads. They can be provided with load-indicating washers to ensure correct tightening; these have dimples formed in the washer face that squash at the required load. They are useful for beam splice connections, but are more expensive than ordinary bolts, so are often overlooked. The mating faces being joined should be de-scaled and not painted, to ensure adequate friction to resist slippage.
I (second moment of area) A geometrical property of a structural section that determines its stiffness (resistance to deflection) in conjunction with its Young's modulus.
I-beam A beam with a cross section in the shape of a capital I. This is efficient as the material is concentrated at the extremes of the section, increasing the second moment of area. The limit of its strength may be governed by lateral torsional buckling, where the unrestrained compression flange is unstable and moves sideways.
I-joists Usually timber elements, assembled to make more efficient use of timber. The web is of sheet material such as plywood or OSB, and the flanges are of timber, glued together. These are generally designed by a specialist manufacturer and there are various proprietary types.
Imposed Load The load imparted to a structure by the objects that it supports, such as people, furniture, vehicles and so on. Other imposed loads are caused by snow on roofs, for example. British Standards define values of imposed load to be used in design for various classes of building, and in ultimate limit state (ULS) design a factor is applied to the imposed load to compare with the design strength of the structure. For deflection calculations, the unfactored load is used.
Joist A beam usually arranged in parallel with others to support a floor, roof or ceiling. In domestic construction these are usually of timber, but steel and concrete joists are also used.
Joist Hanger A metal bracket shaped to hold the end of a timber joist and support it on a wall or beam. Different types are made for each situation and must be fixed according to the manufacturer's instructions to achieve the design capacity. Masonry-supported hangers may require a number of courses of masonry above for stability, and this should not be overlooked during the process of construction.
kilonewton (kN) A unit of force, equivalent to 1000 newtons. This is equivalent to the weight of approximately 100 kilograms (actually 98.1kg). Commonly used in structural design, as the magnitude is appropriate to most projects.
Lime A traditional building material made by burning limestone, chalk (calcium carbonate) and clay in a kiln, then slaking the resulting caustic quicklime (calcium oxide) with water to form hydrated lime (calcium hydroxide). As lime putty, it is mixed with sand to make mortar for bricklaying, which although weaker than Portland cement mortar is more flexible and capable of accommodating building movement. Powdered, as "bag lime", it is added to cement mortar to improve workability and plasticity. Traditional lime mortar and plaster is recommended for older buildings and restoration projects, and re-pointing lime masonry should always be done with lime-sand mortar and never cement mortar. The use of lime is enjoying a resurgence in recent years as its qualities have become better appreciated.
Live Load Another term for Imposed Load.
Moment A measure of force causing a turning effect due an offset distance between the line of action and reaction. This is used to assess problems of bending in beams, or overturning in structures such as retaining walls or dams. A moment is calculated by multiplying the force by the length of the "lever arm" perpendicular to the direction of the force, hence its unit: newton metres (Nm). Magic moments are uncommon in this line of work...
Mortar Material used between bricks or blocks to form masonry structures. In design, it is not considered to act as "glue" but to distribute the pressure between the uneven surfaces of the bricks and allow regular courses to be achieved. Excessively strong mortar is not desirable, as its ability to accommodate building movement is reduced and the tendency for bricks or stone to crack is increased.
Needle Beam A short length of timber or steel that is inserted through a hole in a wall, with props at each end to enable the wall to be supported while work takes place beneath. Typically at 1 metre spacing, this is the correct way to support a wall while a lintel or frame is inserted over an opening. Inevitably, in domestic refurbishment work there is a conflict due to the disruption to the rooms above - for example having to remove the bathtub to allow the needle through the wall!
Neutral Axis (NA) The axis within a beam where the axial stresses induced by bending are zero. In a symmetrical section this is half-way through the depth of the beam, between the extremes of compression and tension. In an asymmetric section or a composite, such as an RC beam, this is not the case.
newton (N) The basic unit of force, named after Sir Isaac Newton. Defined as the force needed to accelerate a mass of 1kg by 1 m/s2, but in structural terms 9.81N being the weight of 1kg (due to gravity) is more useful.
Padstone A piece of masonry designed to spread a concentrated load (such as the support reaction from a beam) into a structure. Commonly these are made from concrete either cast in-situ or pre-cast, or engineering bricks. In domestic building, we find that these are frequently built incorrectly, and the design codes are not easy to interpret either, whereas a steel beam is comparatively difficult to install wrongly.
Party Wall A wall between adjoining buildings that is shared by both owners and to which both have rights of support. The Party Wall Act governs the rights of owners in terms of making alterations to Party Walls (and also floors, in multiple-occupancy buildings such as flats), and provides methods of achieving agreement. Helpful guidance is available from the government in their booklet available to download from
Portal Frame A structural frame whose purpose is not only to support gravity loads in a vertical direction, but also to resist horizontal loads such as wind loads and sway forces. These are increasingly adopted in domestic projects as the desire to have wider openings in walls means that the inherent stability of the box-like form of the house walls can no longer be relied on. A portal frame will have full moment resistance at the corners, either mitred and welded or with a bolted haunch connection. Structural Engineers Cambridge Ltd have developed a design that is compact and safe to erect on site, avoiding the bulky protrusion that a bolted haunch can dictate.
Quoin One of the cornerstones of a masonry building, often in stone or contrasting brick to improve strength or for decorative effect.
Raft Foundation A special type of foundation used to cope with weak or variable ground conditions, allowing the weight of the building to be spread over a larger area than conventional footings. Usually constructed of reinforced concrete, with edge and intermediate beams and a floor slab. The best shape for a raft foundation is square or rectangular with a length to breadth ratio not exceeding 2 to 1; L-shapes or T-shapes increase the risk of bending and twisting due to ground settlement.
Rafter One of a series of parallel inclined beams supporting a roof covering, usually of sawn timber. The design of rafters is similar to that for beams or joists, except that of course the load is not perpendicular to the axis of the member.
Reinforced Concrete (RC) A structural material employing steel bars to improve the strength of concrete, which is very weak in tension on its own, to form a composite. In an RC beam, the concrete is considered to resist the compressive stresses and the steel resists tension, and the design codes have been developed to prevent the crushing of the concrete or the width of cracks in the tension zone as the steel bars stretch under load. It has good resistance to fire and corrosion, as the steel is protected by the concrete cover, and in restricted sites the ability to assemble structures from individual bars and fluid concrete is an advantage.
Rolled Steel Joist (RSJ) A range of steel sections that has not been commonly available since the 1980s but the term is still colloquially used for any steel beam. They are identified in older structures by their tapered flanges and heavier section than modern Universal sections.
Serviceability The state in which a building or structure can operate properly. The serviceability limit state is the point at which deflection or movement of the structure under working loads should not impair the usefulness, safety or weather-tightness of the building.
Shear Is the effect caused by a pair of opposing parallel forces. It is often difficult to visualise in structural concepts, but at its most basic a rectangle subjected to a shear force would distort into a parallelogram.
Shear Centre The position of a beam's axis where an applied load will not induce torsion (twisting). For symmetrical sections, the shear centre coincides with the centreline but in asymmetric sections such as channels this is not the case. The shear centre for a channel section lies outside the back face, opposite the toes.
Simply Supported An idealised model of beam design, in which the beam is considered to be resting between "knife edges", so that no moment can be transferred between the beam and the supports. The beam is thus free to rotate at the supports. The opposite is the "built-in" (or encastré) support, which is considered rigidly clamped so that rotation is resisted. The distribution of bending moments and stresses is quite different, and the deflection is also altered.
Soil In engineering terms, soil refers to all types of earth or ground interacting with structures, and comprises mineral particles of rock, sand or clay together with water and air. Of interest is the bearing capacity of the soil; its ability to support the weight of a building without excessive settlement or shear failure.
Span The distance between the centres of supports for a beam or truss. This is greater than the width of the opening but less than the length of the beam. Where spans are quoted in design calculations this should not be taken as the length of the beam!
Splice Connection A means of assembling a longer beam or column on site using bolts and plates, or welding. The engineer will design the connection to resist failure by slippage of mating faces or by fracture of the components. Critical connections in steel will use HSFG bolts (see above) as ordinary high-tensile bolts are not intended to prevent slippage at design loads.
Stability The ability of a structure to maintain its shape or position with variations in applied load. An unstable structure is undesirable, as the changing shape of the structure causes the point of application of force to move, thus increasing the adverse effect on the structure.
Stress Is the measure of internal force within a body, defined as the amount of force per unit area. This is contrary to the psychological meaning, which describes the symptoms rather than the cause. For engineers, stress is not harmful – it's the strain that's the problem!
Strain The measure of how much a material deforms under stress, i.e. the symptom. In tension, the strain is shown by the material stretching, for example. Each material will have a limiting strain at which interatomic forces can no longer resist the applied stress, and either plastic deformation or rupture occurs.
Structure A system of components devised to support loads and/or resist forces. This includes buildings, but also bridges, dams, frames, aircraft, cobwebs and skeletons, for example.
Subsidence The downward movement of the ground, either due to compaction from overloading or to shrinkage. In clay soils, shrinkage will occur when the moisture content of the soil reduces, such as in a drought or due to the growth of trees.
Torsion A force causing twisting. In beams, an offset between the applied load and the shear centre of the beam will induce torsion, which I-shaped sections are not well disposed to resist; tubes are much more efficient.
Truss A structure composed of straight members connected in triangles, requiring no bending resistance in principle as the member forces are purely axial. Roofs and bridges are applications for which trusses are well suited.
Trussed Rafters The most common method of producing modern roofs, in which timber members are joined by toothed steel plates and erected on site by crane. Assembly in factory conditions allows higher design stresses to be allowed and economy of production. Trussed roofs do not normally require support from internal walls and can span 9 metres or more between supports.
Ultimate Limit State (ULS) The condition in which the factored loads are compared with the capacity of the structure. The working loads are multiplied by factors so as to provide a margin of safety against failure, as opposed to the allowable stress method which reduces the calculated strength to compare with the working loads. Modern design prefers the ULS method to provide greater versatility in applying partial factors to each aspect of differing load cases and material properties.
Universal Beam (UB) The range of standard steel beam sizes, having an I-shaped cross-section. These are efficient at handling bending forces about their major (stronger) axis, but if not restrained against lateral torsional buckling can be limited in their capacity.
Universal Column (UC) A range of steel sizes with a cross-section like a horizontal capital H. Intended mainly for columns (vertical members), they are also useful as beams where lateral restraint is not available, as their strength in the minor (weaker) axis is greater than for a UB section.
Wind-Moment Frame A type of structural frame that is simpler to design but less resistant to horizontal forces than a portal frame. Used where wind or sway loads are not critical. The top member is designed as a simply-supported beam and the connections to the columns are designed with a smaller capacity to resist bending moments.
Yield The point at which the stress in a material exceeds its ability to behave elastically, that is to recover to the original shape on removal of the applied load.
Young's modulus (E) The relationship between stress and strain in an elastic material. For engineering purposes, the ratio is considered constant within a certain range. An elastic material will return to its original shape once the stress is removed, and even materials such as concrete or glass are elastic to a degree. Once the yield stress is reached, plastic strain occurs from which the material cannot recover. A stiff material such as steel has a high Young's modulus, while rubber has a low modulus.
Z (Elastic modulus) A geometrical property of a section that determines its bending strength. Defined as the second moment of area divided by the distance from the neutral axis to the extremity of the section (Z=I/y). Thus, the stress due to a bending moment is inversely proportional to Z.