Exploring the effects of clip flexibility on the behavior of standing seam diaphragms to brace cold-formed steel purlins

Cold-formed steel C- and Z-section purlins in standing seam roof systems rely on the diaphragm action provided by the panels to restrain lateral movements. The clip connection between the purlin and the panel has inherent and sometimes intentional flexibility designed to accommodate thermal deformations. Although this flexibility has major implications on the transfer of diaphragm forces throughout these systems, this effect is not typically incorporated into analytical models and, as a result, the models often grossly overpredict the demands on the diaphragm and the subsequent impacts on the purlin behavior.

To demonstrate the influence of the clip connection flexibility on the distribution of forces throughout a system of purlins, a series of shell element finite element models are presented. The flexibility of the clips connecting the purlin to the panels is varied from rigid, to linear elastic to bi-linear elastic-plastic. Initial two-purlin models are calibrated to the mid-span lateral displacements of purlin systems tested according to the AISI S908 Base Test. The same clip flexibilities are then utilized in larger models of simple-span and multi-span purlin systems with multiple purlin lines and dispersed bracing anchors. The variation of the diaphragm forces relative to the variation in the clip flexibility is presented. More importantly, because much of the system behavior is linked to the action of the diaphragm, the additional effects on brace forces, purlin lateral bending and torsion moments are investigated and compared to existing analytical models. The comparisons demonstrate the need to improve the existing analytical models to better predict purlin system behavior.

Learning Objectives:
At the end of the presentation, the attendee should be able to explain the changes in the diaphragm forces that result from introducing flexibility in the clip connection.

View content