Simplified design method for cold-formed steel portal frames with back-to-back channel sections

Cold-formed steel (CFS) portal frames, typically consisting of back-to-back channel sections with bolted eaves and apex joints formed through brackets, are commonly used in industrial, agricultural, and commercial buildings in New Zealand and Australasia. In design practice, their design is often simplified by assuming rigid joints, which can underestimate frame deflections by a factor of two (or more), and underestimate the bending moment at the apex by as much as 40 %. Furthermore, particularly for column design, shear lag effects at the eaves are ignored, overestimating strength by around 20 % to 40 %. A combination of these simplifications by practicing engineers can lead to premature failures in the connection zones and potentially result in unsafe design outcomes. This paper presents, for the first time, a simple beam idealization method that engineers can use to predict frame capacity. A detailed non-linear finite element (FE) model of a CFS portal frame is developed and validated against experimental results. Using the validated FE model, the ultimate failure load of a frame can be predicted (assuming that the brackets do not fail first). The validated FE model is then used to assess the influence of joint flexibility, moment capacities of the eaves and apex brackets, and the reduced moment capacity of the channel section due to the shear lag effect in the eaves joints. Based on these findings, the accuracy of the simplified design method is assessed. The simplified approach predicts the load capacity of CFS portal frames within 1 % of detailed FE models. • Study on structural behaviour and design of CFS portal frames with back-to-back channels. • Detailed FE models of semi-rigid CFS joints are developed and validated against experimental results. • Influence of joint flexibility, bracket strength and premature buckling of channels due to shear lag effects is studied. • A new beam idealization approach is proposed to consider joint flexibility and shear-lag effects in CFS bolted joints. • A simplified yet effective method was proposed for predicting the gravity load carrying capacity of CFS portal frames.