Lateral buckling of restrained roof purlins

A simple model is developed for the elastic lateral buckling of a continuous cold-formed roof purlin whose top flange is prevented from deflecting laterally, and which is braced against lateral deflection and twist at the supports and intermediate bridge points. The effects of cross-section distortion are neglected, but so are the torsional restraints provided by roof sheeting against twisting. Local buckling is ignored, and must be designed against separately.

The model is analysed by a finite element computer program, and predictions of the maximum moment in a segment at elastic buckling under either uplift or gravity loading are presented graphically.

A simple method of using this elastic buckling information to determine the design working load capacity of a roof purlin is then presented, and this is illustrated by a worked example. An improved method is also presented which allows the restraining effects which occur between adjacent segments during elastic buckling to be estimated. Strength predictions obtained from the improved method are compared with test results.