If cold-formed steel sections are not laterally supported at frequent intervals, the allowable unit stress must be reduced to avoid failure from lateral instability. The amount of reduction depends on the shape and proportions of the section and the spacing of lateral supports. (See AISI Specification for the Design of Cold-Formed Steel Structural Members.)
Because of the torsional flexibility of lightweight channel and Z sections, their use as beams without close lateral support is not recommended. When a compression flange is fully connected to a deck or sheathing material, the flange is considered braced for its full length and bracing of the other flange may not be needed to prevent buckling of the beam. This depends on the collateral material and its connections, dimensions of the member, and the span.
When laterally unsupported beams must be used, or where lateral buckling of a flexural member is likely to occur, consideration should be given to the use of relatively bulky sections that have two webs, such as hat or box sections (Fig. 8.1o, p, and q).
