The Institute is underway with a design guide for these common structures. Portal frames from cold formed members (typically back-to-back channels) share all the structural behaviours of conventional frames from hot rolled steelwork, but are complicated by the influence of the connections. In a frame from cold formed members, the connections at eaves and apex are flexible, so there is a significant redistribution of bending moments around the frame, generally increasing the moment at the apex. The assessment of frame stability and the calculation of deflections must recognise the flexibility in the connection details, as the frame may not be adequate if the analysis is based on rigid connections.
There are therefore two essential pre-requisites; the stiffness of the actual connection detail must be known, and the analysis must be able to accommodate semi-continuous connections. Physical connections may be tested to determine their stiffness characteristics, or these may be calculated by finite element analysis. Commonly available software is generally limited to either pinned or rigid connections, so SCI are developing a software tool that will analyse portal frames, including connection flexibility. The analysis software should be available on the web, in the next few months. This design guide, including a full worked example, will be available in Autumn 2019. It is hoped that this guidance, together with the launch of a “SCI Assessed” scheme for the design and construction of cold formed portals will contribute to best practice in this this type of frame.