Secondary Elements - Steel Structures.

In the normal single-storey building the cladding is supported  on secondary members which transmit the loads back to main structural steel frames.The spacing of the frames, determined by the overall economy of the building, is normally in the range 5–8m, with 6m and 7.5m as the most common spacings.

A combination of cladding performance, erectability and the restraint requirements for economically-designed main frames dictates that the purlin and rail spacing should be 1.5–2m.

For this range the most economic solution has proved to be cold-formed light-gauge sections of proprietary shape and volume produced to order on computer numerically controlled (CNC) rolling machines.These have proved to be extremely efficient since the components are delivered to site pre-engineered to the exact requirements.which minimizes fabrication and erection times and eliminates material wastage. Because of the high volumes, manufacturers have been encouraged to develop and test all material-efficient sections.These fall into three main categories: Zed, modified Zed and Sigma sections. Figure 1.3 illustrates the range.

Popular purlin and frame sections
Fig. 1.3 Popular purlin and frame sections

The Zed section was the first shape to be introduced. It is material-efficient but the major disadvantage is that the principal axes are inclined to the web. If subject to unrestrained bending in the plane of the web, out-of-plane displacements occur: if these are restrained, out-of-plane forces are generated.

More complicated shapes have to be rolled rather than press braked. This is a feature of the UK, where the market is supplied by relatively few manufacturers and the volumes produced by each allow the advanced manufacturing techniques to be employed, giving competitive products and service.

As roof pitches become lower, modified Zed sections have been developed  with the inclination of the principal axis considerably reduced, so enhancing overall performance. Stiffening has been introduced, improving material efficiency.

The Sigma shape, in which the shear centre is approximately coincident with the load application line, has advantages.One manufacturer now produces, using rolling, a third-generation product of this configuration, which is economical.

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