Experimental study of the section moment capacity of cold-formed and screw-fastened rectangular hollow flange beams

This paper presents the results of an experimental study on the section moment capacity of cold-formed steel beams with rectangular hollow flanges. The new Rectangular Hollow Flange Beam (RHFB) is fabricated using a unique manufacturing process in which two cold-formed rectangular hollow flanges are screw fastened to a web plate. It possesses unique stress-strain characteristics and initial imperfections that are different from conventional open cold-formed steel sections and welded hollow flange beam sections. In this study, the section moment capacity of RHFBs was investigated using a series of full scale bending tests. Twenty two section moment capacity tests of RHFBs were conducted on short span flexural members with simply supported end conditions for three steel grades G300, G500 and G550 and varying thicknesses. Test capacity results were compared with the capacities predicted by the current design rules in the Australian and North American steel design standards, AS 4100, AS/NZS 4600 and AISI S100, to verify their applicability to RHFBs. The results show that although the current design rules based on effective width principles predicted the section moment capacities of RHFBs slightly unconservatively in comparison to test results, they can be used for RHFBs provided a suitable screw spacing is used. However, the Direct Strength Method (DSM) based design rules are found to predict the section moment capacity of this new screw-fastened hollow flange beam section conservatively in most cases. This research shows that the DSM based design rules including the new inelastic reserve bending capacity provisions in AISI S100 can be used to predict the section moment capacities of both screw-fastened and welded hollow flange beam sections conservatively.