Structural performance of cold-formed steel
Cold-formed steel sections tend to be more sensitive to local buckling effects than typical hot rolled sections. Cross sections are generally stiffened to improve resistance to local buckling.
The structural behaviour of cold-formed steel can be distinguished from hot rolled section in one important way. The flanges and webs of conventional hot rolled sections are relatively stocky and thick compared to cold-formed steel sections, whilst cold-formed sections are comparatively thin and slender. Sections can be visualised as series of plates connected at the corners. When these steel plates are subjected to compression buckling is likely to occur in the plane of the plate. The tendency to buckle increases as the breadth to thickness ratio of the plate increases. Compressive forces develop in the section either as a result of direct compression in columns and struts or flexural compression in beams.
The corners themselves help stiffen these plates against buckling. Where an edge is unsupported the stress needed to cause buckling reduces considerably. This behaviour can be easily visualised using a sheet of paper. In its flattened form the sheet has no resistance to compression. Form a right-angled fold along one edge and then apply a light compression. The free edge will buckle, the stiffened edge will try to resist buckling. Fold a second edge parallel to the first and repeat the test.
The geometry of the cross section and the position of bends are used to stabilise the section against local buckling effects. Intermediate stiffeners may also be used where the distance between the corners is relatively large to increase the stability of the plate. In some deep purlin sections appreciable compressive forces may develop in the webs as well as the flanges and stiffeners are introduced to improve the stability of these sections.
Acoustic performance
Satisfactory acoustic performance can be achieved with lightweight, cold-formed steel walls.
Wall and floor constructions using cold-formed steel, if properly designed, have been shown to provide good noise reduction characteristics, even though strict application of the mass law would suggest that lightweight stud frame walls would not perform well. The mass law is more appropriately applied to heavyweight, solid constructions. Multiple layer walls with the layers effectively separated perform well in noise reduction. Party walls consisting of two separate frames (similar to party wall construction in timber stud frame) can achieve a weighted sound reduction index of 62. The acoustic properties of cold-formed steel stud constructions are well documented.
Fire performance
Where required, the fire resistance of cold-formed sections can generally be achieved very simply.
Many buildings using cold-formed steel sections as primary load bearing elements such as stud walls and floor require to have a minimum period of fire resistance. Cold-formed steel sections at elevated temperatures exhibit slightly greater strength reductions than hot rolled steel sections. Sprayed protective coatings are not generally applied to cold-formed sections, as plane board systems are more appropriate. Plasterboard linings contribute greatly to the fire resistance. Effective fire resistance of up to one and a half hours can be obtained using two layers of fire resistant board. Fire resistant boards are stronger and shrink less than standard plasterboard. During a fire the joints between boards open, exposing the steel to greater temperatures. The use of two layers with staggered joints affords additional protection to the steel.
