Tests of Cold-Formed Rectangular Hollow Section Portal Frames (No. R783)

Abstract

This report describes tests on three large scale portal frames manufactured from cold-formed RHS in both Grade C350 and Grade C450 steel. The loading simulated gravity and wind loads. The connections, lateral restraint mechanism, loading method and test procedure are explained. The results indicate that a plastic collapse mechanism was formed in each frame. There was no failure associated with any of the joints. Simple plastic analysis provided a reasonable estimate of the ultimate load, even though the analysis did not include second-order effects or non-linear material properties. An advanced analysis which included second order effects, material non-linearity, and member imperfections slightly over-predicted the strength of the frame, since the analysis underestimated the deflections and hence the second-order effects, as the analysis did not consider the small loss of connection rigidity. A second-order inelastic analysis without member imperfections provided the best estimates of the strengths of the frames. The distribution of curvature throughout the portal frame was analysed. The results indicate that for the portal frames tested a rotation capacity of R = 4 is adequate to form the plastic collapse mechanism. Although the cold-formed RHS do not satisfy the material ductility requirements for plastic design, there was no failure associated with lack of material ductility in the three frames tested, demonstrating that the restriction on cold-formed RHS from plastic design is unwarranted provided that the connections are sufficiently ductile for any required plastic rotations.