Experimental investigation of apex connections in cold-formed steel portal frames under wind uplift

This paper presents an experimental investigation of the rotational stiffness and capacity of three types of cold-formed steel portal frame apex connections under uplift loading: Type A connections with web bolts only, Type B connections with additional top flange bolts, and Type C connections with a haunch. The connections were tested in four-point bending. Failure consistently took place in the rafter by cross-sectional instability at the bolt pair connecting the rafter channels back-to-back nearest the connection. However, the capacity of the Type A connections was on average 11% lower than that of the other types, due to the presence of a shear lag effect. Adding bolts to the top flange (Type B) slightly increased the initial stiffness of the connection (by 12%), as well as the final stiffness during the bolt bearing phase (by 40%), but the most significant improvement in moment-rotation behaviour originated from a reduction in the length of the slip plateau. Type C connections displayed the highest initial stiffness and showed no low-stiffness slip phase. When compared to nominally identical specimens under gravity loading, no significant differences in behaviour were detected for the Type A and B connections. The Type C connections, on the other hand, were stiffer under uplift loading, with a 37% higher initial stiffness and a 21% higher final stiffness.