Research Spotlight: Design Method for Columns with Intermediate Elastic Torsional Restraint

2017 Annual Stability Conference Presentation

Session S7 – Stability of Columns
Thursday, March 23, 2017
1:15 pm

Design Method 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. Frames used for shelters over large areas have unbraced columns, and for larger spans, a knee brace is required to transfer the large bending moment from the rafters to the columns. The knee brace to column connection creates an intermediate elastic torsional restraint on the column as the column is partially restrained against twist rotation where the knee brace joining the column is connected. 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 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.

Hannah B. Blum and Kim J.R. Rasmussen, University of Sydney, Sydney, NSW, Australia