Current affairs

Floor joists are 150 to 300mm deep, depending on the span required. Different cross sections can be combined to add strength, sometimes incorporating hot rolled elements to accommodate a difference in use, such as to provide ground and first floor open space for hotel conferences and communal areas, as opposed to the smaller requirements of bedrooms. All steelwork should be CE marked to EN 1090. The NHBC and other warranty providers require a

‘warm frame’ construction for external walls, where insulation external to the steel frame stops condensation and corrosion. Galvanising can last 250 years and cope with one off flooding. Cladding is commonly brick, with render or rainscreens other options, while different systems are used for high rises. He described a typical floor build up: an acoustic

systems is dependent on what the BRs state and the requirements of the building itself’. He continued: ‘As a combustible material, timber will never achieve A1 or A2 classification, so in a non combustible world, timber doesn’t really have a place’. He added that Eurocodes will probably be updated in 2022 and include provisions for CLT, ‘but currently wall build ups used in buildings will need to have appropriate fire resistance periods according to the EN standards, to show they actually meet the BR requirements.’

Light steel structures Andrew Way of the Steel Construction Institute (SCI) said light steel (LS) structures are primarily used in multi storey construction up to 12 storeys – some being multi occupancy. To that height, light steel can provide the full load bearing structure, aided by cross bracings across the wall panels to provide stability. With additional support (eg an extra concrete course;

hot rolled steel beams or posts), it can go higher in modular construction, which is often used for student accommodation and hotels where rooms are repeated, but also in low rise where steel frames are fitted out off site with board, insulation and potentially cladding. In the single frame option, resilient bars are placed between the steel studs and plasterboard linings. The system’s light weight suits, for example, residential developments built over large supermarkets, and its speed of construction lends itself to educational and medical buildings. Commonly, cold formed steel cross sections are

used for infill walls, either stick built on site or panelised. Wall panel sections are typically 70 to 150mm, but an increased gauge of steel may be needed for high rises.

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floor system on top of structural board (oriented strand or plywood board is normally used atop the LS joist, and gypsum based board for fire resistance and acoustics). There may be insulation in the cavity. In multi occupancy buildings, LS joists or concrete are often used to give more solidity between adjacent apartments, though LS joists can still satisfy standard fire and acoustic requirements for separating floors, using spaced horizontal joists spanning wall panels. Individual elements can be built, then boarding applied on site, or floor cassettes used for speed. Composite floors are formed on a profiled

steel deck, the steel pre galvanised, then cast in situ in concrete around 130 to 200mm deep. Propping requirements depend on the spans and depth. Sometimes extra reinforcement of the slab is required to prevent cracking, or for fire resistance of typically 90 minutes or two hours. Mr Way said more information is available on the SCI’s website, as are guidance documents and information sheets. A new publication on LSF and fire resistance is expected in 2019.

Insulating concrete forms Nudura’s Jean-Marc Bouvier traced the origins of insulating concrete forms (ICF) to Canada in the 1970s. The product is a hybrid using timber and light gauge steel, depending on environmental and building requirements. Components are assembled on site to the desired height, filled with concrete, then services added. The air barrier is the concrete, and indoor air quality is maintained because ‘polystyrene doesn’t propagate moulds or fungus and condensation doesn’t penetrate the wall’. Cladding can be render, brick or aluminium. It’s a simple system that’s flexible, easy to work with

and a quick build. For multi storey accommodation, six people can build a floor every ten days. One school was built by five people six months ahead of schedule, as building can be done in all weathers. Other projects included passive houses, multi storey residences, care homes, student accommodation, hotels, hospitals and offices. Emphasising the good thermal resistance and air tightness ‘inherent in the vibrated concrete’, and

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