Web crippling instability response in CFS built-up open beams: numerical study and design

The adoption of cold-formed steel (CFS) in structural systems has risen significantly in recent years due to various favourable features such as cost-effectiveness, speed of construction, and lightweight structures. As a result, CFS members are suitable for modular construction. Beams are primary structural members responsible for transferring loads from the floors to the adjacent columns. Built-up I-sections composed of two channel sections are preferred as flexural members owing to their higher torsional rigidity and other stability characteristics over monochannel sections. The webs of CFS I-beams are vulnerable to web crippling deformation under the influence of localized loading due to their higher sectional slenderness. The past research on web crippling studies has mainly focused on CFS mono-sectional profiles. However, the web crippling instability response of a single CFS channel section may differ from that of an open built-up beam composed of two such channels. Limited findings on CFS built-up sections have been reported so far. Hence, the current web crippling design rules may not be adequate for CFS open built-up beams across a wide range of parameters and need to be explored in detail. The current study focuses on the web crippling response of CFS built-up I-beams composed of two plain channel sections fastened through the web at various distinct points across the cross-section and longitudinally. First, a numerical model was developed in ABAQUS, and validated against the relevant test data available in the literature. The validated model was used to carry out an extensive parametric study by varying critical parameters used in the web crippling design expression of the North American Specification (AISI S100). The effect of all these critical parameters on the web crippling behavoir has been explored. Lastly, the web crippling design strengths were determined using the current North American Specification (AISI S100) and Eurocode (EN1993-1-3). These web crippling design strengths were compared against the numerical web crippling strengths to assess the accuracy of the current design codes. Both these design codes showed inconsistency in the predictions, as they over-predicted the web crippling strengths in some cases and under-predicted in others. This clearly reflects the need for more studies on such built-up beams to bring out accurate design rules for the same.