AbstractThis paper proposes a novel, cold-formed portal framing system that comprises tapered box members formed from two cold-formed nested channel sections. The proposed method possesses structural and nonstructural advantages, such as improved seismic performance, enhanced building hygiene through bird and dust resistance, further corrosion resistance, and aesthetic improvement, as a result of fly bracing removal. The novelty of the box member lies in the tapering ability of the section, leading to material and paint savings. To examine the failure mechanisms and structural performance of the tapered box portal frame and to investigate the adequacy of the design method, two full-scale portal frames with an 18.16-m span were built and tested to failure under two common loading scenarios. The first was lateral cyclic loading into the inelastic range in conjunction with a vertically acting permanent load. The possibility of ductile plastic hinge formation in a severe earthquake was investigated using lateral cyclic testing. The second was vertical loading to failure. A novel economic loading setup was used to apply the gravity load on the frame. For the seismic design of this section type, slenderness limits are proposed for seismic applications along with the design ductility. A rotational stiffness value for a nominally pinned portal frame column base was also suggested in this study.