Design Methods for Columns with Intermediate Elastic Torsional Restraint

Cold-formed steel haunched portal frames are popular structures in industrial and housing applications. They are mostly used as sheds, garages, and shelters, and are common in rural areas. Cold-formed steel portal frames with spans of up to 30 m (100 ft) are now being constructed in Australia. As these large structures are fairly new to the market, there is limited data on their performance and inadequate design guidance and recommendations. In the specific frame system analyzed herein, the column is partially restrained against twist rotation at an intermediate point where the knee brace joining the rafter and column is connected, and is otherwise unbraced. Current design guidelines do not directly account for the restraint provided by the knee connection and require the determination of the member effective length. Due to the variations of the column base stiffness and rotational restraint of the knee connection, the column effective length is difficult to quantify. Therefore, a new design method is proposed in this paper which eliminates the need to determine the effective length. The design capacity is calculated using the Direct Strength Method with inputs from a column buckling energy analysis. Internal actions are determined using a calibrated beam finite element model with notional horizontal forces, and the interaction equation involving bending and compression is utilized to determine the column strength. A reliability check is completed and the results compared to experimental frame ultimate loads. It is shown that the frame strength determined from the design method presented herein is a suitable method for the design of columns with an intermediate elastic torsional restraint in haunched portal frames.

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