Steel Construction Glossary

See Also: Glossary of Steel Terminology

Axial Load. A load, in tension or compression, straight along a member.
Beam. Any member which can span a gap and resist loads in bending. In steel, such a beam is usually I shaped, with flanges top and bottom joined by a web.
Bending moment. Defined as (force x distance) it is a situation that tends to bend structural members. The strength of a steel member to resist bending is called its bending moment resistance. The amount that a member bends is its stiffness.
Bi-axial bending. A steel member can be bent in one plane, and simultaneously be bent at 90 degrees, so is bent about both axis.
Blind Pop Rivets. These are rivets which do not have a hole from inside to out after fixing.
Bolts and Nuts. Most steel structures are fastened together on site by nuts and bolts. These come in a variety of sizes, from 6mm to 100mm in diameter. They can be soft (grade 4-60, or high tension 8-80, or very high tension 10-90: the figures refer to the ultimate strength and yield strength of bolts.For example a 4-60 has an ultimate strength of 400N/mm2 but yields at 60% of this, 240N/mm2; an 8-80 bolt has an ultimate strength of 800N/mm2, and yields at 80% of this, 640N/mm2.
Bolts can act in tension (pulling) and in shear (being sliced by two sliding plates). Bolts can also be designed to work in friction, by squeezing plates so hard together that they cannot slip (these are called friction grip bolts, and sound good: but it is extremely hard to get them all to the right tension, and they should normally be avoided). In general, structural bolts are 8-80.
Bracing. Steel members need to be kept in the right geometry to resist loads, and this geometry is usually maintained by bracing. Bracing can be a diagonal member, forming a triangle with 2 other members, and keeping them in the required position. These braces, or bracing members, are usually tubes, either square or round, with bolted connections at the ends. If a diagonal member is not convenient (perhaps because it would cross a door opening) then the diagonal bracing may be replaced by beams with fixed end connections which would do the same job, of keeping the members in the right geometry. Individual beams carrying purlins or rails may have knee bracing from the zed member to the opposite flange to stop the beam from twisting.
Bridges. Please visit www.steel-bridges.com for a full glossary of bridge terms.
Cantilever. Member which is only fixed at one end, carrying, for example, an overhang.
Cladding. The external envelope of the building, particularly of the walls. (See sheeting, curtain walling, sandwich panels).
Codes and Standards. Steel frames are designed to various national and international codes. Of these BS5950 is probably the best. It is relatively simple to use, and gives safe and economical designs. The new Eurocode for steel was originally based on BS5950, but then some sauerkraut, paella, frog’s legs, goulash and other tasty additives were spooned on, and the Eurocode is a dog’s dinner, to be avoided; it is difficult to use and adds cost. There are loading codes, too: BS6399 is the British code, and is excellent except for 6399/2, the wind code: although pretending to be based on real tests on real buildings, much of the wind code is faked, and has many prescriptions which are very different from real test results. Unfortunately, the false prescriptions of 6399/2 were adopted in Eurocodes and even in International codes. The old British Wind Code, CP3 ChV Pt2 remains safe and economical. Bridges can be designed to BS 5400, which is highly conservative and gives very safe structures.
Column. Any vertical member which resists an axial load. In steel a column is usually H shaped, very like an I beam except that the flanges are wider, which helps it to carry an axial load without buckling. Or a column can be a rectangular or square or round hollow section.
Combined axial and bending loads. A member can be in compression or in tension, and at the same time, be bent.
Containers. International shipping is usually by Container. 40-foot and 20-foot containers are the most common. These can take items 11.9 or 5.85m long, and this dictates the sizes of members. It is difficult to load and unload steel from containers, unless they are open-top. These have a canvas roof so you can crane items in and out. But open top containers are less common than General Purpose, GP, closed containers, so we do need a bit of notice for large shipments.
Composite Decking. Please see Shear Connector below first, then read on. Decking on which concrete is to be poured can serve first as formwork, to make the shape that the concrete must take, without too much sagging; but the decking can have a number of deformities formed into it, and these serve as shear connectors, bonding the steel deck and the concrete together, making them stiffer and stronger. The decking serves as the steel reinforcement in the tension zone of the concrete slab.
Crane. Overhead travelling crane. Electric overhead travelling crane (EOT). A lifting device which can be installed in a building. It has a lifting unit (a hoist) which is mounted on a ‘crab’ which can move from side to side on a crane bridge (which may be a single girder or a double girder). The bridge is mounted on end carriages which can travel along crane beams anywhere in the building, but usually along the two sides. Where crane bridges are exceedingly long (such as in an aircraft hangar), it is necessary to suspend the bridge from the roof on a number of suspension carriages. The hangar then has to be designed to carry the crane loads. REIDsteelmanufacture such ‘hangar cranes’ as part of the hangar design where needed. Most users would be better served using ground mounted mobile cranes, rather than hangar cranes, because the crane makes the hangar higher, and heavier, and more expensive and requires great accuracy in erection. But if the crane is needed, then the crane and hangar must be designed as an entity by REIDsteel.
Eaves. The sides of a building which are horizontal, and often have a gutter.
Elastic design. In this method of design, the steel remains ‘elastic’, that is to say that it never reaches yield stress with the factored load applied; or only reaches permitted stresses under unfactored working loads. This may sound good, but it leads to more expensive design; and can also lead to collapse if overloaded, unlike plastic design.
Fibreglass. See insulation.
Flashing. Edges of sheets, at the corners, around doors, above a masonry wall and so on are usually covered with a profiled stell sheet called a flashing.
Frames. Frames make up the structural skeleton of a building, and the purlins and the siderails are fixed to the frames with bolted cleats. Frames are usually spaced at regular distances along the length of a building. There is a wide variety of frame types. A frame can be an I beam which is simply supported on two walls. Or it can be a truss, usually of triangular shape, and this triangular shape may itself be stiffened in a series of triangles (triangulated) to make it keep its shape, and be stiff, and carry loads. Very frequently, a frame is made of beams which slope to meet at the centre of the roof with a bolted connection; and these roof beams, or rafters, can be connected to the vertical members with bolted connections. Often they are connected to make a stiff strong joint with the columns, so becoming continuous (with moment connections). This frame type is called a portal frame. The columns are often stronger and heavier, which can result in the rafter being lighter. The connections then have to be stronger than the rafter, as strong as the columns; so the lower ends of the rafters are made deeper with a triangular insert called a haunch. Similar haunches may also be used at the apex, or any other beam to beam joint.
Gable End. Usually a building has two long sides which are horizontal, but the roof usually has a slope, or two slopes, to drain rainwater down to a gutter on one side, or to 2 gutters, one either side. The other two sides, or ends, have a slope or slopes, and these ends with slopes are the gable ends. The triangle above the level of the side eaves is called the gable peak.
Gable posts. The gable end of a building can be very wide, and gable posts are needed to resist wind loads, to support sheeting or masonry. The top of the gable posts produce a sideways force into the building, which is resisted by gable end bracing systems.
Hangar (Hanger). The origin of the word ‘hangar’ is a Northern French dialect word for a cow pen or barn. But the English term, hangar, usually refers to a wide span building, mainly for aircraft, airships or perhaps large boats. The term came into being during the first world war, for obvious reasons. REIDsteel made the first commercial hangars, originally for Louis Bleriot, who crashed into a field and used one of our existing barns to mend his aircraft. The next day he ordered ‘trois hangars pour Lamotte Beuvron’, and REIDsteel have been designing and making hangars ever since. The term ‘hanger’ is just a mis-spelling, but is very common.
Hangar Doors. These are big wide sliding doors. They are often mounted on multiple tracks, so that they can open into a stack, giving a wide opening. Because they are invariably wide and heavy, hangar doors are almost always bottom rolling on big wheels (the bigger the wheels, the easier to roll the door slabs open and shut). If the whole opening is needed at any one time, then the doors will have to bunch up outside to the left or the right or more usually both; they are then said to have outriggers. The doors can be electrically driven or hand driven with gears and a turning handle; if electricity supply is unreliable, better to choose manual doors, which are easy to operate. Wide doors can have threshold-less panic doors or even roller shutter doors for access and exit. If there is no space for outriggers, and yet the full hangar width is required to open, try our Megadoor. Please visitwww.reidsteel.aero for a full description of hangar doors.
Haunches. Steel inserts, usually triangular in shape, which are used to make connections between beams, often at the connections of rafters to stanchions, but can be used at any beam to beam connection.
Hoist. A lifting device which can be installed in a building to lift weights. A hoist can be suspended from a strong point in a building; or it can be on a trolley on a monorail, running in a straight line anywhere in the building. A hoist can be manual or electric.
I Beam. See beam above.
Insulation. The sheeting of a building often has insulation against heat, cold, fire or noise. Codes can stipulate a degree of thermal insulation of roofs and walls, called the ‘U Value’. The U Value is measured in Watts per degree centigrade per square metre. The U Value should take into account not just the insulation capability of the main sheeted area but also the edge conditions and thermal bridging. There are crude and cheap ways of getting some insulation by means of a blanket spread over the purlins and rails and trapped under the sheeting. The blanket could be bubble wrap, sometimes coated with paper (called sisalisation) or fibreglass with a coating (paper or plastic or foil or reinforced foil). All of these are squashed down over the purlins or rails where thermal bridging occurs. Most of these would burn and drip in a fire situation. It is very difficult to eliminate leaks round the edges of the blankets, and they need to have ‘wings’ to staple together or tape together. The only one of these ‘blanket’ solutions that is marginally acceptable in a manned building is the one with reinforced foil skin, with staples and tape; but even this is not very good because of thermal bridging where it is crushed. Another solution is to use stiff panels of insulation, either a foam sandwich panels with reinforced foil skins, or stiff rockwool panels; these are much better than blankets because they do not crush, but the edges often need a supporting grid, and always need sealing. A better system is to have 2 layers of sheeting, with a spaced air gap between them. The spacers are designed to minimise thermal bridging. The insulation layer, which can be anything you choose, but is usually fibreglass or rockwool, is placed between the skins with no crushing. Rockwool is more expensive to buy and to transport and to install, but can enhance fire resistance. The two skins can both be sealed so there are 2 roofs. All double skin constructions give a degree of sound insulation: the two skins do not touch; they both have different resonant frequencies; and the insulation mat damps the vibration. The next step is for foam or rockwool filled sandwich panels, with galvanised and coated inner and outer skins. Some visual, erection and strength benefits may come with sandwich panels, say for office walls. But all forms of sandwich panel are more expensive to procure and transport than the equivalent double skin system, and they are much less good sound insulation.
Joist. A light beam. Often joists are the secondary members carrying floor loads to the main structural frames.
LCA. Lifecycle Assessment
Load Factor LF. Steel is designed to specified loads with an added safety factor to ensure that it does not fail under its design loads. Under current British standards, live loads are augmented by 60% (LF1.6) and wind and self weight loads augmented by 40% (LF 1.4). Other codes have slightly different Load Factors
Loading. All buildings are designed to resist loads. Normal roof load is 60kg/m2, 0.6kN/m2. An office floor is designed for 2.5kN/m2 of live load; a apartment building for 1.5kN/m2; grandstands for 400kg/m2; stores may be designed for any level of loading depending on use. Wind loads are calculated knowing the wind speed, generally the 3 second gust once in 50 years at a height of 10m; wind loads can vary from about 0.5kN/m2 to 250kg/m2 in hurricane areas. Wind loads increase with height. Loadings are adjusted with load factors and coefficients shown in codes and standards.
Louvers. These are vents in the walls of buildings, usually with inclined slats to let air in but keep rain out. Preferably they should be fitted at low level, say 1.2m above the ground, so that the area used by workers gets ventilated. They work best in conjunction with ridge vents. In areas subject to sandstorms, the vents on the windward side should have closers. In areas that are cold as well as hot, all the louvers should have closers.
Megadoor. In an aircraft or ship building hangar, where the whole width is required to be open at the same time, and where there is no space for outriggers, then buy a REIDsteel hangar with a Megadoor fitted. Megadoor is a Swedish Company with whom we have arrangements to produce complete hangars, or new doors on existing hangars. The Megadoor is on the same principal as a venetian blind, but fabric covered on both sides of the horizontal slats; it is opened by lifting the bottom beam and the slats bunch up above the opening. Wide doors, greater than about 28m, need to have gate props dividing up the opening, and folding away when the doors are open.
Mobile Elevating Work Platform MEWP. A device mounted on 4 wheels with a hydraulically operated extending boom with a caged work platform at the top of it. It is to provide safe access to anywhere above the 2m level, and is an essential erection tool where ever there are regulations.
Nuts and Bolts. See bolts and nuts above.
Overhead door. This is a door made of a number of wide steel panels joined together horizontally. They are fitted into vertical guides at either end which allow them to slide up and down. The panels enable the door to bend in a curved track above the opening. The door can be pulled up this track, or allowed to slide down it, using hand chains, or a motor, or both, to open and shut the door.
Packs. Steel is manufactured to fairly close tolerances, but not to precise tolerances. It is normal in a steel structure that some misfit of members may occur. For this reason frames are often made with slightly under-sized members, and an allowance is made for packs, thin steel plates, to be inserted into the gaps to ensure good fit.
Plastic Design. Steel when overstressed will stretch or compress or bend permanently with no loss of strength (this permanent deformation is ‘plastic’ deformation). In a steel frame, a steel beam may bend slightly, and in doing so redistribute load to other parts of the frame, leading to a safe design method: provided that the frame does not fail at the connections, or by buckling; and that the frame remains elastic at unfactored working loads, plastic design offers the safest and most economical steel design method for many frame types.
Plastic Hinge. When a steel member is stressed beyond its elastic limit, it can bend plastically with no loss of strength; the place in the beam where this happens is a ‘plastic hinge’.
Pop Rivets. These are rivets which are pressed into a drilled hole, and are then squashed into a shape with a special tool from the outside, squeezing 2 skins of sheeting together (used for side laps in sheeting). Only blind rivets should be used.
Portal Frame. A frame where the rafters are strongly connected to the outer columns (or stanchions), usually with haunches.
Prop. Any vertical member which supports a frame and carries axial load downwards.
Propped Portal. A portal frame can span effectively over spans of 5m to 60m or so, but it becomes heavier with bigger spans. With spans above 24m or so, it is more economical to introduce a central prop; and with bigger spans, more props can be introduced.
Purlins and Rails. The sheeting carries the climatic loads and any other loads applied to the envelope of the building and is fixed onto purlins (on the roof) and onto rails (side rails or end rails) on the walls. Purlins and rails then resist the loads from the sheeting, and carry these loads to the main frames of the building. Purlins and rails can be of various sections: historically, they were hot rolled joists (I sections), or channels (C Sections), or angles (L sections) (equal leg angles or unequal leg angles). But these sections are quite heavy, and tend to bend under their own weight, or under thermal stresses left in from the hot rolling process. It was also difficult to fix sheeting on to the hot rolled members, because the flanges of these tend to be quite thick. It was found that much thinner steel could be bent into shapes which could carry the sheeting loads, and these (cold rolled or press-braked) sections could be stiffer and stronger and lighter and easier to fix sheeting to than the previous hot rolled sections. The shapes that are chosen are C shaped channel sections, or Zed shaped sections. It was then further found that, because the sections could be very thin, the metal would tend to buckle prematurely, and for this reason many of the simple Z or C shapes are further deformed with more bends, making them stiffer. Because these sections were very thin, they need protection against rusting so are usually galvanised. While they are being rolled or bent, it is easy to punch holes into the webs or flanges of the sections, and these are used to bolt them to the frames or supports. Because steel load bearing members are stronger and stiffer if they are continuous over a number of supports, instead of simply supported (pin-ended), the sections are usually slightly asymmetrical so that one bit can fit neatly into another bit of the same profile but upside down. In this way purlins and rails can be made continuous by overlapping them over the supports; or can be joined over supports using short lengths of the same section upside down, called sleeves. Because purlins and rails are much stronger in their deep direction, but very weak in their narrow direction (try bending a ruler, thin way and deep way), it is often necessary to brace them on their weak plane with sag ties and adjustable rods, to make sure they do not bend or twist.REIDsteel purlins and rails have been load tested, and safe design criteria are available.
Radius of Gyration. A measure of the resistance of a member to buckling under axial load, measured in cm.
Rafters. The bits of frames which carry the roof loads.
Ridge Capping. Where the two ends of roof sheeting come together at the apex, there is usually a gap, which is covered by a profiled sheet capping. This capping has fillers to give it a snug fit against the sheeting to prevent rain or birds from entering.
Ridge vent. There is nowhere better to provide ventilation than at the ridge. The simple ridge vent is a hole at the ridge, covered by a cowl and bird mesh. If fire venting is needed, that too is far better at the ridge, as hot smoke rises. Vents down the slope need special water-proofing, and lead to expense and maintenance and possible future leaks. Good natural ventilation also needs low level louvers.
Rivets. In older times, rivets were the principle method of fixing 2 steel plates together. Holes were drilled or cut or punched; then a red hot slug of metal was placed in the holes, and the rivet was held at one end by a heavy hammer, and bashed on the other end with another hammer, widening the rivet until it completely filled the hole, and forming a head so that it clamped the plates. Rivets made a tight join as they shrunk on cooling. Now most rivets that are still used a lightweight and usually pressed together cold.
Roller Shutter. This is a door made of a number of narrow steel flats joined together horizontally. They are fitted into vertical guides at either end which allow them to slide up and down. The narrow slats enable the door to be rolled up around a tube at the top. The tube can be rotated either with hand chains, or a motor, or both, to open and shut the door.
Sag ties. See purlins and rails.
Section Modulus. The section modulus of a steel member multiplied by the yield stress determines its bending resistance. The section modulus is calculated knowing the geometry of the section and is measured in cm3.The elastic modulus (Zx or Zy) is a measure of the bending resistance up to the elastic limit, and the plastic modulus (Sx or Sy) is a measure of its ultimate resistance against bending.
Second Moment of Area I. The second moment of area of a member is a function of its geometry, and is a measure of the stiffness of the member, in other words, its resistance to bending. The bigger and thicker the member, the greater is its stiffness. I is measured in cm4.
Self drilling self tapping screws. Most often, sheeting is fixed to thin purlins or rails by means of self drilling self tapping screws. With a special screw gun, the screw first acts as a drill to cut a hole through the sheet, then penetrates the hole and taps a screw thread through the purlin; then finally cuts a slightly wider hole through the outer sheeting to create a seal.
Shear Connector. In a concrete-and-steel construction, the assembly is stronger and stiffer if the steel and the concrete act together in bending; if the concrete were simply placed on the steel, it would slide along the surface of the steel in bending; so shear connectors are welded to the top of the steel and embedded in the concrete, forcing the steel and concrete to act together. Shear connectors are usually steel studs, about 3 inches long by about ¾ wide, with a knob on the end. They are welded to the steel at the other end with a stud welder.
Shear force. A force acting on a bit of material so as to slice through it; or to make it deform by lozenging. A bolt between two plates which are sliding past each other is subject to a shear force. A top of a column, to which a loaded rafter is attached, is changing shape from a rectangle to a lozenge shape because of shear force.
Sheeting. The outer envelope or skin of the building is the sheeting. This is usually a profiled metal sheet in steel or aluminium. It is usually profiled, that is, bent, to have a series of ridges and valleys, so that it has the strength and stiffness to span between supports (purlins or rails). It can thus resist wind, or snow loads and walking loads. The profile can be a series of curves (sinusoidal) or a series of flat areas between sharp bends (trapezoidal) or flat pans between two vertical upturned edges making rectangular channels. If steel, the sheeting may be protected from corrosion by zinc coating (galvanised), or a zinc/aluminium coating (aluzinc). Any sheeting may have coatings to give colour and add to protection by waterproofing; this may be on one side only, or on both sides. Even when the exposed side has the main coating, the reverse side may have a thin backing coat, for protection. In many corrosive situations the sheeting will corrode from the underside, and double sided coatings will extend life.
Sliding door. This is a door that runs on an upper and lower track, and opens from side to side. It can be top-hung, on rollers, in a track, if the door is small. But a wider door is usually better if it rolls on a ground track, which carries its weight, while the upper track simply guides the door top horizontally; this is because the weight of the door deflects the supporting steel downwards, especially if the opening is wide and the door slab is heavy.
Smoke venting. In the event of a fire, it may be necessary to allow the smoke to vent. If this is required, then the only place suitable for venting is at the ridge. Sometimes people do specify vents halfway down the slope; but hot air always rises and vents are more efficient at the apex. A permanently open ridge vent (see ridge vent) may be sufficient. But if not venting can be achieved with softening roof lights; or with automatically opening vents. REIDsteel can supply either type, or we can provide the openings for you to install your own vents.
Stanchions. The outer columns of a building, usually fixed to the rafters.
Stress. Stress is force per unit area. It is normal to use Newtons per square mm, N/mm2. A Newton is the weight of an apple: there are 10 Newtons per kilogramme. Normal steel resists 275 N/mm2 or 355N/mm2. All steel is tested and is usually stronger than the specified strength.
Tied Portal. A portal is a frame where the connections of the rafter to the columns provide bending moment resistance, and the bending moment at the eaves give a horizontal force at the feet. In a tied portal, a horizontal tie between the columns at eaves level reduces the horizontal force at the feet, and provides more moment resistance in the rafter at the eaves. This has the effect of reducing bending in the rafter from downwards loads.
Truss. Single beams can carry loads but beams become heavy, expensive and bendy over long spans. Instead, rafters or other frames can be made with steel top and bottom chords; and between these, steel members can be placed to form a series of triangles, which stiffen and strengthen the frame. This sort of frame is a truss. The truss can be rectangular, or triangular, of paraboloid in form. Trusses can span between 2 supports or over any number of supports.
Welding. Welding consists of heating metal on two adjacent plates and then adding new welded metal between them to make a molten pool; when cooled the pool becomes solid and a strong bond is made. A fillet weld is a weld made in an angle between two plates. A butt weld is made when two plates are end to end, with a gap between them, and the whole gap is filled with molten weld material, in one or more passes. Welding can be done with heat from burning gas (Oxy Acetylene), or with heat from electrical discharge between a welding rod and the metal parts. Because of the high heat in welding, the weld must be protected from oxidising, either with the flux supplied on the rod; or by a stream of inert gas around the weld site.
Yield Stress. If steel is stretched, it gets longer. If compressed, shorter. At first it gets changes length ‘elastically’, that is to say it goes back to its original length when released from the tension or compression. But at some stress level, the deformed steel does not return exactly to its original length: it has deformed first elastically, then plastically. The steel can carry on being deformed, with no loss of strength, but it no longer goes back to the same length. The stress at which the some of the deformation becomes permanent is the Yield Stress. Funnily after stretching most steel gets stronger, and when it breaks, it does so at the Ultimate Stress. Grade 50 steel’s Yield Stress is about 355N/mm2, but its ultimate stress is a lot higher, 500N/mm2. Grade 43 yields at about 275N/mm2 and fails at 430N/mm2