Experimental investigation of locally and distortionally buckled portal frames

The paper presents an experimental investigation into the structural performance of cold-formed steel pitched portal frames subject to local and distortional buckling prior to global sway failure. The columns and rafters were constructed from back-to-back lipped channel sections, and the apex and eave joints comprised brackets bolted to both the webs and flanges of the channel sections. Three frames were tested, subjected to different combinations of horizontal and vertical loads. In all tests, lateral and torsional restraints effectively constrained the frames to deform in their own plane. Local buckling developed at a relatively early stage of loading while distortional buckling occurred when the loads were near ultimate. The occurrence of local and distortional buckling reduced the horizontal stiffness and ultimate load significantly. Failure occurred when a spatial plastic hinge formed at the top of the columns in the vicinity of the eave joints. Component tests, including tests on tension coupons and apex and eave joints, were also conducted to obtain the material and connection properties of the frames. A critical appraisal is made of the accuracy of existing design guidelines to predict the ultimate strength of the frames tested experimentally, including the interactive effect of local, distortional and overall sway buckling.