Cold-formed sections as beams - Steel Structures.

In situations where a relatively lightly loaded beam is required such as a purlin or sheeting rail spanning between main frames supporting the cladding in a portal frame, it is common practice to use a cold-formed section produced cold from flat steel sheet, typically between about 1mm and 6mm in thickness, in a wide range of shapes of the type shown in Fig. 16.12. A particular feature is that normally each section is formed from a single flat bent into the required shape; thus most available sections are not doubly symmetric but channels, zeds and other singly sym- metric shapes. The forming process does, however, readily permit the use of quite complex cross-sections, incorporating longitudinal stiffening ribs and lips at the edges of flanges. Since the original coils are usually galvanized, the members do not normally require further protective treatment.

Typical cold-formed section beam shapes
Fig. 16.12 Typical cold-formed section beam shapes
The structural design of cold-formed sections is covered by BS 5950: Part 5,which permits three approaches:

(1) design by calculation using the procedures of the code, section 5, for members in bending
(2) design on the basis of testing using the procedures of section 10 to control the testing and section 10.3 for members in bending
(3) for three commonly used types of member (zed purlins, sheeting rails and lattice joists), design using the simplified set of rules given previously.

In practice option (2) is the most frequently used, with all the major suppliers providing design literature, the basis of which is usually extensive testing of their product range, design being often reduced to the selection of a suitable section for a given span, loading and support arrangement using the tables provided.

Most cold-formed section types are the result of considerable development work by their producers. The profiles are therefore highly engineered so as to produce a near optimum performance, a typical example being the ranges of purlins produced by the leading UK suppliers. Because of the combination of the thin material and the comparative freedom provided by the forming process, this means that most sections will contain plate elements having high width-to-thickness ratios. Local buckling effects, due either to overall bending because the profile is non-compact, or to the introduction of localized loads, are of greater importance than is usually the case for design using hot-rolled sections. BS 5950: Part 5 therefore gives rather more attention to the treatment of slender cross-sections than does BS 5950: Part 1. In addition, manufacturers’ design data normally exploit the post-buckling strength observed in their development tests.

The approach used to deal with sections containing slender elements in BS 5950 is the well accepted effective width technique. This is based on the observation that plates, unlike struts, are able to withstand loads significantly in excess of their initial elastic buckling load, provided some measure of support is available to at least one of their longitudinal edges. Buckling then leads to a redistribution of stress, with the regions adjacent to the supported longitudinal  edges attracting higher stresses and the other parts of the plate becoming progressively less effective, as shown in Fig. 16.13. A simple design representation of the condition of  Fig. 16.13 consists of replacing the actual post-buckling stress distribution with the approximation shown in Fig. 16.14. The structural properties of the member (strength and stiffness) are then calculated for this effective cross-section as illus- trated in Fig. 16.15. Tabulated information in BS 5950 for steel of yield strength 280N/mm2 makes the application of this approach simpler in the sense that effective widths may readily be determined, although cross-sectional properties have still to be calculated.The use of manufacturer’s literature removes this requirement. For beams, Part 5 also covers the design of reinforcing lips on the usual basis of ensuring that the free edge of a flange supported by a single web behaves as if
both edges were supported; web crushing under local loads, lateral–torsional buckling and the approximate determination of deflections take into account any loss of plating effectiveness.

Forzed purlins or sheeting rails section 9 of BS 5950: Part 5 provides a set of simple empirically based design rules.Although easy to use, these are likely to lead to heavier members for a given loading, span and support arrangement than either of the other permitted procedures.A particular difference of this material is its use of unfactored loads, with the design conditions being expressed directly in member property requirements.

Loss of plating effectiveness at progressively higher compressive stress
Fig. 16.13 Loss of plating effectiveness at progressively higher compressive stress

Effective width design approximation
Fig. 16.14 Effective width design approximation

Effective cross-section
Fig. 16.15 Effective cross-section

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