Steel Buildings in Europe

Part 1: Architect’s Guide 1 - 30 4.2 Secondary steelwork Secondary beams are used when the spacing of the main beams or trusses is too large for the cladding or roof panels to span between them, or where the cladding spans parallel to the main beams, which is usually the case with pitched roofs. For these secondary members, there is a choice between cold-formed and hot- rolled steel sections. The profiles of typical cold formed sections are shown in Figure 4.2. A cold formed section can be up to 30% lighter than a hot rolled section. 1 2 3 4 C profile ℓ max = 10 m 140 mm < h < 300 mm  profile ℓ max = 12 m 140 mm < h < 300 mm profile ℓ max = 16 m 250 mm < h < 420 mm Z profile ℓ max = 12 m 120 mm < h < 400 mm Figure 4.2 Typical cross sections of cold formed beams Cold formed sections are manufactured from galvanized steel and this normally provides sufficient protection against corrosion in the internal environment of the building (an exception might be, for example, in aggressive environments such as cattle sheds, where ammonia is present). Secondary members of cold-formed sections are used at relatively low spacing, typically between 1,6 m and 2,5 m. Very long secondary members can be fabricated as small trusses. 4.3 Roofs The choice between a flat roof and a pitched roof often depends on the particular preferences in the local or national region. Some countries favour flat roofs that are able to sustain significant imposed loading, other countries favour pitched roofs that facilitate drainage and which are subject to only very modest imposed loading. Clearly, the type of cladding that is appropriate depends on those choices and circumstances. 4.3.1 Pitched roofs The slope of a pitched roof also depends on local circumstances and custom. A slope of at least 10% (6°) is normally provided. Where profiled sheeting is used, the profiles run down the slope, to facilitate drainage. Insulation must therefore be below the outer sheeting (possibly as a composite panel). The sheeting is supported on purlins spanning between the