Design Approach of Cold-formed Steel Portal Frames Abstract A structural arrangement of a portal frame that is easy to fabricate and assemble is developed. The rafter and column members are formed from single cold-formed channel sections, which are bolted, back-to-back at the eaves and apex joints, and connected to the foundation through angle irons. Variables in the frames include the span, width of the channel flanges and the strength of the channels. The design of the frame members follows the traditional approach of applying the effective width method to control local buckling, followed by an assessment of cross-sectional and buckling requirements. However, the yield and overall buckling requirements are modified by a factor of 0.8 to account for stress concentrations, shear lag and bearing deformations in the connections. All deflections of the frames considered are found to be within the limits provided in the literature and by the two codes of practice. It is proposed that the frames of the structure should be provided in a kit form and erected in “meccano” fashion。53205

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1. Introduction

Cold-formed steel portal frames have been developed as a viable alternative to traditional hot-rolled I-sections,especially for spans less than 15 m. The rafter and column members are formed from single channel sections, which are bolted, back-to-back at the eaves and apex joints, and connected to the foundation through angle. Thin sections are used so that the members can be lifted into position without using sophisticated machinery. The spans of the frames investigated ranged from 10-14 m, with a constant eaves height of 3 m and a pitch of 10ο. The portal frames

are delivered to site in sections cut to length and with connection holes pre-punched at the factory as part of the manufacturing process. The product developed in this

investigation provides greater flexibility in structural composition without the requirement of a sophisticated workshop, employing only standard commercially available cold-formed steel profiles. Tests carried out on these frames have shown the column and rafter members to have considerable buckling strength  (Dundu and Kemp, 2006a, b). Increased strength is a result of orienting adjacent column and rafter channels in opposite directions, to take advantage of the counterbalancing

moments and forces in the back-to-back connections. Three modes of failure were observed namely: local buckling of the compression zone of the flange and web of the channels, lateral torsional buckling of the channels between points of lateral support and bolts in bearing. The source of failure in all structures was local buckling of the compression flange and web. This occurred after considerable rotation of the channel sections within the eaves connection. Local buckling was

made more critical by stress concentrations, shear lag and bearing deformations caused by back-to-back bolted connections. Calculations, based on SANS 10162-2 (2005), showed that a 300mm deep channel section with different flange widths would be appropriate for a range of purlin spacing. This code is based on the Canadian Code, CAN/ CSA-S16.01 (2001). A typical portal frame built from the cold-formed channels is shown in Fig. 1 and the corresponding connections are given in Fig. 2.

2. Structural Form Development Prior to designing, the structural form had to be

resolved. The structural form includes the span, eaves height, pitch of the roof and type of cladding. At the conceptual stage, it was suggested that three portal frames spanning 10-12 m be considered for investigation. The spacing of the frames, determined by the overall economy of the building, is normally in the range 4.5-8 m, with 6and 7.5 m as the most common spacings for mild steel frames. Since the frames under investigation are made of steel that is prone to local buckling, it was decided that all three frames be spaced at 4.5 m centers. The structural performance of a portal frame is sensitive to the eaves height. The eaves height should be at a