Application of thin-walled construction
1. INTRODUCTION
The general trend towards lighter and more economic forms of steel construction has led to increases in the use of cold-formed, light gauge elements. With the global trend to renewable energy production, driven by solar panels (PV or thermal), the growth in the use of cold-formed profiles for solar parks and solar modules erected on existing roofing has been spectacular in the last decade, and this trend is expected to continue.
The same trend with e-commerce growth and the need for need automated warehouses, including high-bay racking systems, can also be seen.
These elements may be either members or flat products.
Members such as purlins, sheeting rails, and main framing components may be used in place of heavier and more costly hot-rolled products or substituted for alternative materials such as timber or aluminium. Flat products such as wall cladding, roof sheeting, and floor decking are often used most effectively by combining them with other materials.
The main advantages in using cold-formed products are:
i. A very wide range of components are available through the flexibility provided by the forming process.
ii. Long lasting duration and strong corrosion resistance through the use of previously metallic coated material and the new generation of Zinc-Alu-Magnesium ZM coating or organic coated steels - also painted on Z and more and more on ZM substrate.
iii. The possibility to provide very high strength steel grades, aligning with steel industry production capabilities in terms of dimensions.
iv. Good quality surface finish, available in a variety of different types.
v. The ability to provide high levels of thermal and acoustic insulation when used in conjunction with suitable insulating materials.
vi. The availability of relatively simple fastening methods, several of which may be used on site.
vii. High ratios of load-bearing resistance to structural weight, especially when combined with high strength steel grades.
viii. Permits the use of considerable off-site prefabrication.
Cold-formed products are usually produced from the original coils of thin sheet material by either:
- Roll-forming in which the coil is progressively unwound and various shapes made in a continuous process by passing the sheet through a series of rolls that progressively develop the required shape (see Figure 1) or
- Folding in which pre-cut lengths are bent to form a suitable structural element.
Whilst the first of these methods is suitable for large volume production, e.g. cold-formed profiles for purlins or rails, the second is more appropriate for low volume production of items that need to be tailored for specific applications, e.g. flashings used to seal the joins between components.
1.1 Available products
1.1.1 Profiled sheeting
Profiled sheeting is available in a wide range of geometries and may be adapted for several different uses.
In single or double skin roof sheeting (see Figure 2), the steel sheet is normally used as the external profile. Its main function is to ensure the security of the roofing. The main function of the internal sheet in a double skin roof is to support the insulation although by incorporating suitable perforations it may assist in improving the acoustic properties of the building. Double skins may use the same profile for both sheets - in which case the roofing is generally called 'parallel texture' - or it can use a different profile internally, thereby replacing the purlins. In this latter case, the arrangement is termed 'crossed texture.'
For flat roofs (see Figure 2b), the steel profile has, in addition to supporting its own dead weight, the function of carrying the insulation and the maintenance and climatic (snow and wind) loads.
In such arrangements it is important to ensure that the insulation can safely span the troughs between adjacent upper flanges of the corrugations.
For vertical walls, the outer skin of either a single or double wall cladding (see Figure 3) has to ensure air tightness and water tightness of the vertical walls, whilst at the same time providing an attractive visual aspect to the building. The internal skin is then effectively a structural tray or cassette, spanning horizontally between columns that provides a flat internal wall and supports the insulation.
With the evolution of thermal regulation in all EU countries to improve the energy efficiency of buildings, requests for insulation thickness increase have changed dramatically over the last two decades, and the double skin solution has been largely substituted by the insulated sandwich panel solution. Regardless, it has remained popular for the renovation of existing buildings where the existing single skin roof or cladding could serve as the basis for a double skin solution to improve insulation.
Trapezoidal sheeting may also be used for flooring, e.g. in prefabricated houses.
Sheeting may also be used as permanent shuttering to concrete, in which case it may be designed merely to support the weight of the wet concrete and any construction loading. A more efficient arrangement is, however, to use the steel sheeting in the final condition to also produce composite action in the floor, as shown in Figure 4.
Because precoated sheeting is used for these flooring arrangements, the lower face may be used to provide a sufficiently visually attractive ceiling in some cases. Organic coated steel can be a useful solution here when used, for example, in open car parks where it can contribute to the visual appearance (one colour per storey) and corrosion resistance.
Since many steel cladding and roofing profiles are also available in equivalent geometrical shapes manufactured from translucent material, it is normally quite straightforward to create areas of natural lighting in buildings using this arrangement.
In order to properly fix cladding and roofing panels, it is necessary to employ a range of smaller components at the various junctions. These products are termed flashings and will frequently need to be provided in a range of shapes and sizes. The products may sometimes also fulfil additional functions, e.g. to act as gutters. The folding operation is ideally suited to the production of this multiplicity of different items.
1.1.2 Members - light gauge steel structure
Cold-formed members are available in a very wide range of shapes and sizes. Since they are normally formed from a single sheet, the cross-section should be such as not to require additional jointing, e.g. an I-section can only be formed by joining two channels back to back at the web. Figure 5 provides some idea of the range of products currently available.
The mechanical characteristics of light gauge steel profiles should be determined according European standard EN 1090 - 4 - Execution of steel structures and aluminium structures - Part 4: Technical requirements for cold-formed structural steel elements members and sheeting and cold-formed steel structures for roof, ceiling, floor, and wall and cladding applications.
As EN 1090 is a harmonised standard according to CPR – Construction Product Regulation (Regulation (EU) No 305/2011), CE marking is mandatory for all of these construction products.
1.1.3 Sandwich panels
As previously stated, in the last two decades, the use of prefabricated insulated sandwich panels (see Figure 6) formed from two metal sheets with a foam core has increased considerably.
This has clearly been a consequence of energy efficient building regulations at the EU level and in member states' construction regulations. Such an arrangement, which may be installed very rapidly, is particularly suitable as a means of providing thermal insulation and mechanical resistance at the same time as the basic weather shield. It consists of two metal faces bonded to an internal layer of rigid PUR/PIR foam. Such panels can be installed very quickly, thus saving time on site.
This is a prefabricated roofing and facade solution covering a wide range of requirements: facade aesthetics with a wide range of colours and finishes thanks to prepainted steel, long term durability and corrosion resistance, mechanical resistance, fire reaction and fire resistance performances (at least to a certain extent), thermal insulation and air tightness performances, and productivity on site.
