Thin-walled members and sheeting
To introduce cold-formed members and to discuss their manufacturing, applications, and design.
This lecture introduces cold-formed sections and members. It discusses methods of manufacturing and applications and shows how these sections have certain advantages over more conventional steelwork. The design methods generally used are explained and advice is given on practical considerations.
1. INTRODUCTION TO THE DESIGN OF COLD-FORMED SECTIONS
Originally, the use of cold-formed thin-walled steel sections was mainly confined to products where weight saving was of prime importance, e.g. in the aircraft, railway, and motor industries. Simple types of cold-formed profiles (mainly similar to hot-rolled shapes), as well as profiled sheeting, have also been used as non-structural elements in building for more than one hundred years.
Systematic research work, carried out over the past six decades, as well as improved manufacturing technology, protection against corrosion to ensure long durability, increased material strength, and the availability of codes of practice for design, have led to wider use of cold-formed sections within the building industry. In many countries, cold-formed steel construction is among the fastest growing branch of the structural steel market.
1.1 Typical products and uses
Cold-formed sections are prismatic elements of constant sheet thickness formed by a sequence of plane sub-elements and folds in order to perform specific load bearing functions for members and also sometimes for space-covering functions (see Figures 1-3).
A characteristic feature of cold-formed sections is that slender parts in compression are stiffened by folding (intermediate and edge stiffeners), which delays or prevents premature buckling of the compressed zones. This phenomenon is discussed in Section 2.
The types of products available for use in building structures are:
- linear members, mainly used in the higher range of thickness, as structural sections for comparatively low loads on small spans (purlins and rails), as columns and vertical supports, and in trusses
- plane load-bearing members in the lower range of thickness and with load-bearing resistance are used as flooring, and roofing and cladding solutions where a space-covering function under moderate distributed loading is needed
The use of cold-formed structural members offers many advantages:
- the shape of the section can be optimised to make the best use of the material
- there is much scope for innovation (in practice this has proved to be very significant) including the use of high/very high strength steels
- cold-formed members combined with roofing and cladding solutions offer very competitive and reliable solutions which provide a space-covering function and lateral restraint against buckling. Light-weight industrial buildings constructed from cold-formed members and sheeting are an example of the combination of these two effects (see Figure 4).
These advantages can, therefore, be generally classified as weight-saving, by optimising the products with respect to the load-bearing function and constructional demands, and functional in terms of space-covering ability.
Cold-formed sections can be manufactured either by folding (see Figure 5), press braking (see Figure 6), or cold rolling/profiling (see Figure 7). Cold rolling/profiling is by far the most efficient and competitive solution.
For small quantities of building elements with lengths ≤ 6m (in exceptional cases ≤ 12m), it could be advantageous to use hydraulic folding or press-braking machines. The effort required to form the shape depends on the sheet thickness, the ductility of the material, and the shape of the section, which is limited by the strip width.
These manufacturing methods allow the sections to be shaped for optimum load-bearing resistance, intended purpose, and further product processing.
The type of steel used should be suitable for cold-forming. For cold-formed sections and sheeting, it is common to use cold-rolled continuously galvanised steel with yield stresses in the range of 250-280-320-350-390-420-450N/mm2 and with a total elongation of at least 10% for a 12.5mm wide strip, referred to a gauge length lo =80mm, and a ratio of ultimate tensile strength to yield stress of at least 1.1. Recent developments beyond those values – up to 700 N/mm² – have been introduce to answer weight savings request for applications like solar park framing solutions.
Regarding durability and corrosion protection, for typical indoor applications (e.g. purlins in light commercial buildings) zinc coating Z275 (275g/m2) or zinc-aluminium coating ZM120 (120g:m²) is sufficient. For outdoor applications and more corrosive environments, improved protection using suitable coating systems (up to ZM 620 – 620 g/m²) may be necessary and are available in up to 6mm steel substrate thickness.
1.6 Effects of cold-forming
Cold-forming techniques allow the geometrical properties of a shape to be readily varied. It is possible, therefore, to influence the load-bearing behaviour of the element with respect to strength, stiffness, and failure modes by, for example, the introduction of intermediate stiffeners or by ensuring adequate width-to-thickness ratios in adjacent flat parts of the section.
As cold-forming of the steel sheet involves work hardening effects, the yield stress, the ultimate strength, and the ductility are all locally influenced by an amount which depends on the bending radius, the thickness of the sheet, the type of steel, and the forming process. The average yield stress of the section then depends on the number of corners and the width of the flat elements. The effect of cold forming on the yield stress is illustrated in Figure 8.
The average yield stress can be estimated by approximate expressions given in the appropriate codes. In the example, the average yield stress ratio fya/fyb ≈ 1.05 and the corner yield stress ratio fyc/fyb ≈ 1.4.
During the cold-forming process varying stretching forces can also induce residual stresses, which can significantly change the load-bearing resistance of a section. Favourable effects can be observed if residual stresses are induced in parts of the section which act in compression and, at the same time, are susceptible to local buckling.
The development of lightweight construction requires the availability of adequate fastening techniques. Suitable fasteners are bolts with nuts, blind rivets, self-tapping screws, self-drilling screws, and powder actuated fasteners (see Figure 9); industrialised production spot welding and adhesives may also be used. In order to use fasteners in building construction, it is necessary to be familiar with the behaviour of the connections and to lay down design criteria for serviceability and stability. Comprehensive experimental and theoretical investigations form the basis of the analytical evaluation of the load-bearing behaviour of the fasteners under static and dynamic loading. Figure 10 shows fields of application and the appropriate failure modes.
Generally, failure modes causing sudden failure of connections should be avoided. Local over-stressing is indicated by large deformations and should be reduced by load transmission to adjacent fasteners.
Extensive research and product development in the past has led to national design specifications for cold-formed sections and structures in many countries. European Recommendations for the design of cold-formed sections have been developed by the European Convention for Constructional Steelwork [1,2] and form the basis for Part 1-3 of Eurocode 3 'Design of steel structures - General rules - Supplementary rules for cold-formed members and sheeting' . This document, published in 2006, is now under revision.Read more