[ ; The section has a web and two flanges; the web is usually about twice the depth of the flange width.
Circular Hollow Sections: Are hollow steel tubes that are round. CHS. Hot rolled tubes are made from red-hot flat which is rolled into a circle then welded along a seam; Cold rolled tubes are made from a cold flat which is forced into a circle and welded. This has consequences as the cold rolled tube has large built in stresses so is generally not so good as hot rolled.
Cladding: Roof sheeting; Wall sheeting. The sheeting is made of thin steel sheeting which is made able to resist wind, snow and walking loads by being bent (profiled) into a variety of shapes, usually trapezoidal. The sheeting can be single skin, which stops wind and rain but has no insulation value.
Insulation is often needed; it can be achieved with double skins separated by spacers, and in the spaced air gap some insulant, fibreglass, fire resistant mineral wool, foam boards. Insulation can also be provided by sandwich panels which have similar double skins but injected with foam (polyurethane or poly-isocyanurate).
The outer cladding can be of many types: plain Aluzinc, which is cheap and quite resistant to corrosion; polyester enamel coated galvanised which is coloured but only suitable for unaggressive climates; prisma coated steel which is generally very good in hot sunny climates; plastisol coated, which is humidity resistant but not great in intense sun, Ultra Plastisol which is excellent in all environments.
In very wet and saline environments the sheeting will corrode from the underside and so double sided Prisma or double sided ULTRA are worth the extra cost for long life. The liner used for the double skin system is white enamel galvanised, profiled like the outer sheet and walkable on the roof; the wall liner can be thinner.
Columns: Usually steel sections in the shape of H ; The section has a web in the middle and two flanges. The width of the flanges is usually very similar to the depth of the web.(Universal Columns UCs may be say 152 by 152 or 305 by 305; HEA, HEB profiles).
Container Dimensions: Virtually all international freight goes in 20-foot or 40-foot containers. These are 2.2m by 2.2m by either 5.85m or 11.9m long. 40′ containers cost twice as much to ship as 20′, and can carry less weight, but of course more volume.
Given that information, it is odd that some specifiers ask for 6m high columns and 6m bays, thus doubling the freight cost; REIDsteel try to take account of shipping realities when designing your buildings.
Doors: Personnel doors are 2.1 m high and either single (900mm wide) or double (1800mm wide). They may often have panic latches and no outside handle; or panic latches internally and locking knobs outside. They are normally steel faced and insulated. They can also be powder coated aluminium with glazed panels.
Vehicle doors can be: either roller shutters, which coil up into a bundle at the door head; or up-and-over, which are made of insulated horizontal hinged panels that run up a track which curves over at the top of the doors; or sliding, which open to one side or both sides from an overhead suspension track; or rolling, which are similar to sliding but run on wheels on ground rails (usually for big wide heavy doors such as aircraft hangar doors). Fabric doors are also a possibility in special circumstances.
Eaves: The top of the side walls, usually defined as the intersect between the plane of the purlins and the side rails.
Eurocodes: Euronormes: EN; a set of codes which has borrowed a fork-full of Sauerkraut, a juicy Frog-leg, a splat of Paella, a sliver of Spaghetti, a slurp of Goulash and various other delights to coerce people throughout the continent into a uniform dog’s dinner.
Flashings: where bits of sheeting meet, at doors, at windows, at corners, at ridges, at the join between wall sheet and masonry, there is always a need for flashings, purpose made shapes of coated steel to trim the edges.
Floors: (mezzanine floors; suspended floors). Many buildings have suspended floors, either small office floors, sometimes whole production floors over the entire area, on one or more levels.
It is often helpful for these to be framed in steel. The most common is to have columns; and over those columns, horizontal beams; and between these beams, joists at close centres (say 2 to 4m centres); and on these joists, composite galvanised decking on which concrete can be poured to produce a strong light floor.
Sometimes the decking can carry the weight of the wet concrete but sometimes it may need propping while pouring. The floor slab may will need anti crack mesh and may need additional reinforcement bars.
Floors can also be made with prefabricated prestressed Hollow Core concrete planks spanning 4 to 10m over main beams. Light floors can be made with tongue-and-groove plywood or chipboard on very closely spaced joists which may be Zed sections.
Footings: Where the stanchions, props, gable posts come to ground. The steel puts load onto the ground, downwards from the weight and applied loads, up wards from wind, and with sideways thrusts due to the nature of the framing and the applied sway loads.
In countries without frost, the footings are normally at 150mm below the floor slab; in places with frost they should be at least 300mm below floor level; in saline conditions they should normally be above floor level; and there are many required elevations for other reasons.
The footings can be plain square or rectangular pads of concrete to spread the load onto the soil at a depth where it is deemed strong enough; or they can have stubs from buried pads; or they can be on a grillage of ground beams to spread load evenly over a bigger area; or they can be on concrete pile caps on steel or concrete piles where the ground is weak.
The combination of the weight of the pad, structure, walls and slab must be heavy enough to resist wind uplift. Where the soil is reasonably strong (100kN or 10 tons per square metre, say) we can help with the footing design; but for weak soils or special conditions, a local soils engineer should design them.
Galvanising: Steel can be protected from corrosion by galvanising, instead of shot blasting and priming.
In this process, the steel is dipped in acid to remove all the rust; and then it is plunged into a bath of molten zinc; the zinc reacts a little with the steel surface; when it has been in the bath for a calculated time, it is withdrawn and the zinc hardens, leaving a thickness of zinc(usually specified as 85 microns, or 610grammes, per square metre).
The zinc gives excellent protection, by forming a skin of oxide over the exposed surface, and if corrosion does occur, the zinc is eaten away first, leaving the steel intact. Many thin steel items are pre-galvanised; that is coils of steel are unwound, cleaned, rolled through a molten zinc bath, cooled and re-coiled; this gives a good even coating of zinc, but usually much less than the hot dip galavnised finish (135 grammes per square metre is more typical) so do not expect the same life from thin sections with thin zinc coating as you would expect from thick steel with thick coating.
Particular care must be given to cold rolled box sections; or to large multi-component assemblies, as the hot liquid metal can increase stresses and sometimes rip the bits apart.
Gable Posts: Usually the end of a building rises to a ridge, and this is called the ‘Gable End’. The rafter on the gable end is supported by the gable posts, which carry the wind loads from the sheeting rails into the bracing structure.
Gutters and Down-Pipes: Gutters may be needed at the lower end of every roof slope. Normally these will be in PVC (if small) or in coated steel. Normally these will be level, and have frequent down-pipes to suit the rain fall.
It is always preferable to have both gutters and down-pipes outside the building, so when the leak or overflow, the rain will still be where it should be, outside. In big, high structures, the gutters will be very large and should be made walkable.
If the gutters are to be Concealed Gutters in the roof instead of outside, then they must be oversized so they do not over-flow, and probably need to be insulated, and must be leak-proof and will be very costly (architects often draw such gutters when others are paying for them but always have external gutters on their own homes).
Where the lower ends of two roofs meet, there will always be a need for Valley Gutters of Boundary Gutters. These have all the disadvantages of Concealed Gutters, with the bonus of higher water volumes, no over-flow possibility, and not only internal down pipes but also internal underground ducting.
Internal down-pipes into underground ducts can be avoided with expensive symphonic systems if the geometry of the building allows. If we have to, we can; but best to avoid valleys where possible with propped portals or ARCHspans with central ridges instead of valleys, for example.
Liquid Metal Assisted Cracking LMAC: Hot zinc, if exposed to a tiny crack in a component which is in a state of high tension, can cut through the metal like a hot knife through butter. Take care when galvanising!
Loadings: A building is normally designed to resist imposed roof loads (for example 60kg/m2 of snow on roof in most of UK) and service loads, things that people may hang from the building (say 5 or 10kg/m2) and self weight (whatever it is). With these loads a nominal sideways sway load is added for integrity.
The building may also resist other known loads, such as a crane. The building must also resist wind: the sensible criteria is the 3-second gust at a height of 10m with a return probability of 50 years; most airports have this figure; and the exposure can be completely exposed, exposed with scattered obstructions, edge of towns, city centres. The speed may vary from 130km/hour up to about 260 or 280km/hour in hurricane or cyclone or polar areas.
In earthquake areas a building must also resist a horizontal acceleration of a proportion of dead and live loads; perhaps 5, or 10, or 15, or even 20% of the other loads. Floors in buildings also have an imposed live load: for an apartment 150kg/m2; for an office 250kg; for a seated grandstand 400kg/m2; for a corridor or stairs or dance halls 500kg/m2.
Storage might be anything from 500kg/m2 upwards, up to 500kg/m of available height. Partitions, surface finishes, suspended ceilings and other services will add to the dead load.
Load Factors: The design loads are multiplied by factors which ensure that the building is well strong enough for any potential over-load; live loads may be multiplied by 1.6 or 1.5; dead loads may be multiplied by 1.4 or 1.3; Wind loads may be multiplied by 1.4 or 1.5; all depending on the regulations in place. An accidental load or earthquake load may have a load factor of 1.05.
Louvres: These are ventilators to be placed at low level in the walls and consist of coated galvanised steel profiled blades (or aluminium blades) set in frames which can be mounted in masonry or our sheeting. They may need bird mesh or insect mesh. They should normally be between 1m and 2.1m from floor level. (See Ridge Vent below).
As a guide, the area the area of louvre openings should have twice the area of the ridge vent opening.
Natural Lighting: Translucent: This can be translucent fibreglass reinforced plastic sheeting profiled to be the same as the cladding sheets. Like the cladding, it can be single skin or double skin. It can be on the roof or in the wall sheeting.If in the roof, it has to be strong to ensure that no-one falls through it, which makes it expensive;
roof-lights also give solar heat gain and conduction gain, lose heat in cold weather, are the first bit of any roof to degenerate in strong sunshine, are the weakest in strong winds and are the primary source of leaks; on the other hand, translucent sheeting in hit-and-miss pattern as high as possible in the 4 walls are virtually free and suffer much less from the other problems.
Something like 5% of the total plan area should be translucent in hot sunny countries; 10% in temperate zones.
Natural Lighting: Glass: Glass lets in more light and looks better than translucent sheeting. Excellent quality double or triple glazing has much better thermal conduction than translucent sheeting; but glass is expensive and still lets in enormous amounts of solar heat gain. REIDsteel will advise the best use of Glass, and Sunshading.
Plastic design: Steel has the ability to deform without losing strength. A normal frame with connections between members can be loaded elastically and deflect, but return to its original shape; but it can be loaded a lot further, safely, if it is allowed to deform plastically under factored loads.
A condition is usually that the frame is not allowed to deform plastically under un-factored loads. Plastic design gives greater economy and greater safety.
Portal Frames: These are steel frames usually consisting of 2 vertical Stanchions (usually Beam or perhaps Column sections) with rigid connections to a rafter (usually a Beam section). The rafter can be either horizontal, or sloping one way (monoslope), or rising to a ridge (Pitched).
Structurally, when a load pushes down on a rafter, its ends are restrained from bending by the connection to the stanchions. The stanchions are bent, giving horizontal thrusts into the footings.
The portal frame can resist horizontal forces and uplifts, thanks to the rigid connections of rafter to stanchions. The joints at the ends of the rafter may be reinforced with triangular steel pieces called haunches.
Propped Portal Frames: Like Portal frames, these have stanchions and rafters. But they also may have a central prop holding the rafter up, or 2 or more props in each span. The props reduce the strength needed for the rafter to cover large spans.
Because the props are tall and slender and unrestrained, they will usually be square box sections.
Purlins: these are steel (or wood) sections which span between rafters and which carry the roof sheeting. They are often thin galvanised steel Z shaped sections or can be Box Section or beams.
Rails: (Sheeting Rails) These are steel (or wood) sections which span between stanchions or gable posts and which carry the wall sheeting. They are often thin galvanised steel Z shaped sections or can be Box Section or beams.
Ridge Vents and Wall Vents: In hot climates, if people are working in a building, it is good practice for the comfort of the workers to have good ventilation. Hot air rises so to prevent heat build-up high in the building we must let the air escape at the ridge (or Apex).
Then we must make a vent such that wind does not blow rain in to the building; or let birds into the building; and preferably we should make the vent so that, from whatever direction the wind is blowing, it helps drag air out from the ridge.
REIDsteel ridge vents do all this, economically. The ventilation only works if the air comes in at a lower level; and the greater the height difference (the ‘chimney height’ as it is called), the greater the ventilation.
This is convenient as the people are usually working at ground level, so letting the air come in at low level makes them more comfortable. The air can come in by Louvres; or simply by ventilating the blockwork between 1m height and 2.1m height, which is free and works well. Vents at eaves, or leaving the doors open, actually reduces ventilation at low level.
Roof Slope: Roofs should generally slope down towards the outsides to drain water. The more water, then the steeper the roof slope should be. REIDsteel Portal buildings will generally be optimised with roof slopes of 12 degrees.
On very wide buildings, a steep roof slope could give extremely high ridges and gable ends, so it is often better to make the roofs steeper towards the outside, with shallower 3 degree slopes near the apex (so 15 degree, 11 degree, 7 degree, 3 degree slopes may be perfect for buildings up to 200m wide; a Paraboloid Roof). So called ‘flat roofs’ still need a real slope to drain and avoid ponding; 1 degree is the bare minimum, 2 degrees is preferred.
Shipping terms: The normal standard is CFR (carriage and freight) in containers to….; this means that we hire a shipping line container, load it here, transport it to the docks and ship it to the port, where it is unloaded from the ship; the client should pick it up, take it away, unload it and return the container to the port in a short time.
CIF (carriage insurance and freight) is the same but we pay insurance until it arrives in the clients port dockside. Another possibility is FOB (free on board) which ends when we have the container loaded on a vessel, but the client pays from then on, not a very good arrangement. Delays in claiming and unloading will lead to a ‘demurrage’ bill for the client.
Shot blasting and painting: A standard treatment is to put the steel through a shot-blaster which sprays the steel surface with high-speed steel pellets which remove rust and also give the surface of the steel a bit of hardening (peening); then the steel is sprayed with a high build zinc phosphate primer, which will generally protect the steel from rusting during transit and erection (though some impact and abrasion).
Stanchions: The outer vertical columns of a steel structure.
Strength of steel: The strength of steel is generally described as the yield stress, the stress in force per unit area, at which it yields, that is crushes or stretches permanently beyond its elastic limit; (if stressed below its elastic limit, steel returns to its original size when the force is removed; if deformed further than the elastic limit, it will end up longer or shorter than before).
It is a property of steel that it can stretch plastically, beyond its elastic limit, for quite a long way without losing strength; and then even gains in strength. Common steels have a yield of about 275N/mm2 and an ultimate strength of 430N/mm2; or 355N/mm2 with a yield strength of 500N/mm2. (A Newton is the weight of an apple, there are 10 of them to the kilogram). Steel also has shear strength and various resistances to fatigue.
Sun Shading: Sunshine streaming into windows or translucent sheeting obviously let in a lot of heat, however well insulated against conduction. Solar glass may stop a bit of heat gain, but still a lot gets through: obviously better to reduce the area of glass rather than make the glass expensively let in less light.
But if strong sunshine is a problem, the windows or translucents should be given sun shading, especially between 0900 and 1600 hours. If high in the walls, steel sheeting overhangs are the best solution; if small windows in walls, each can have a simple cowl made of coated steel sheet.
But if appearance is important, the simple steel sheeting overhangs or cowls can be replaced with aluminium slatted ‘brise-soleil’ (break-sun) shades, more expensive and more attractive.
Temporary Bracing: during erection, frames and structural components can be unstable without extra bracing, which is only there for erection and can be removed afterwards.
Tied Portal Frames: Like portal frames, these have 2 stanchions and a rafter; but the rafter has a horizontal tie between the stanchions stopping the columns from splaying apart when the rafter is loaded. The tie enables greater spans and reduced thrusts from the columns.
Tied portals may have lighter columns and may be more vulnerable to sideways loads from wind or earthquake, unless properly braced. REIDsteel V-Braced tied portals, or ARCHSPAN frames, are a patented variety of braced tied portals enabling enormous spans.
Ventilation: See ‘Ridge Vent’ and ‘Louvres’ above.
Wind Bracing: Wind on a building gives horizontal loads and uplift loads that must be resisted. The rafters resist the uplift; and sometimes the main frames themselves can resist the sideways sway loading on the stanchions; and sometimes this sway loading is resisted by end to end bracing at roof level.
Wind on the end of the building is resisted by the gable posts, but these then apply horizontal loads into the roof. These loads can be resisted by bracing within the roof space; on small buildings this is at both ends; in longer buildings the bracing to take these loads to ground should be near the centre; in tied or truss frames the bracing should be at tie level.
Sometimes, mezzanine floors can provide the bracing by portalised action of the columns and beams and joists of the floor structure.
Zed Sections: Very light load carrying sections can be made from thin flat coil, which is passed through rollers to bend it into a variety of near Z shapes. These are used for purlins, sheeting rails, and light floor joists.
They are very often made from pre-galvanised steel coil. Sometimes such Zeds are joined end to end with Sleeves, which are shaped to fit the Zed and which add strength and stiffness in multi-span floors.