Torsional restraint of roof sheeting on cold-formed steel hat sections

• Unique test procedure to determine torsional restraint of roof sheeting on cold-formed steel hat sections. • Torsional restraint increases with increase in depth and thickness of hat section and with increase in sheeting thickness. • Finite element flexural-torsional buckling analysis with continuous restraints validated with theoretical solution. • Torsional restraint can significantly increase flexural-torsional buckling capacity. Cold-formed steel hat sections are commonly used as thin-walled beams to support thin profiled roof sheeting. Cold-formed steel sections are usually thinner than hot-rolled sections and have modes of failure which are not commonly encountered in hot-rolled structural steel design. The individual plate elements in cold-formed steel hat sections are normally thin compared with their width and so local and distortional buckling may occur before section yielding. In addition, cold-formed steel hat sections may undergo flexural-torsional buckling because of their low torsional stiffness. For cold-formed steel hat sections which may fail by flexural-torsional buckling, the roof sheeting provides a torsional restraint which can increase the flexural-torsional buckling capacity. Previous research has focused on determining the torsional restraint provided by roof sheeting to C and Z section purlins but no research has been done for hat sections. This paper describes a unique test procedure to determine the value of the torsional restraint provided by roof sheeting on hat sections for 12 different combinations of hat sections and roof sheeting profiles. Three tests for each combination were performed, resulting in a total of 36 tests. The results show that the torsional restraint increases as the depth or thickness of the hat section is increased and also when the sheeting thickness increases. A finite element flexural-torsional buckling analysis is also described for analysing hat sections with elastic continuous restraints, such as those provided by roof sheeting. The finite element analysis is validated with the theoretical solution for the flexural-torsional buckling of hat sections with torsional restraint and diaphragm shear restraint. An example is described to demonstrate the effect of torsional restraint and diaphragm shear restraint on the flexural-torsional buckling of hat sections subjected to uniformly distributed load. It is shown that including these restraints in the analysis will result in a significant increase in the flexural-torsional buckling load and the design load capacity.