The great majority of cold formed beams are (by design) restrained against lateral deflections. This is achieved by connecting them to adjacent elements, roof sheeting or to bracing members. However, there are circumstances where this is not the case and the possibility of lateral buckling has to be considered.
Lateral buckling will not occur if the beam under loading bends only about the minor axis. If the beam is provided with lateral restraints, capable of resisting a lateral force of 3% of the maximum force in the compression flange, the beam may be regarded as restrained and no lateral buckling will occur.
This type of buckling is of importance for long beams with low lateral stiffness and low torsional stiffness such beams under loading will bend about the major axis.
The design approach is based on the "effective length" of the beam for lateral buckling, which is dependent on support and loading conditions.
The effective length of beams with both ends supported and having restraints against twisting is taken as 0.9 times the length, provided the load is applied at bottom flange level. If a load is applied to the top flange which is unrestrained laterally, the effective length is increased by 20%.
This is considered to be a "destabilizing load", i.e. a load that encourages lateral instability. The elastic lateral buckling moment capacity is determined next.
For an I section or symmetrical channel section bent in the plane of the web and loaded through shear centre, this is
A = cross sectional area, in mm
D = web depth, in mm
t = web thickness, in mm
ry= radius of gyration for the lateral bending of section