Structural performance and nonlinear design evaluation of cold-formed steel I-joists with lip-stiffened web openings

Cold-formed steel (CFS) I-joists are increasingly used in long-span floor systems; however, their performance is often governed by serviceability limitations and strength reductions associated with web openings required for building services. While lip-stiffened web openings are widely adopted in practice to mitigate these effects, current design standards do not provide explicit guidance for I-shaped CFS joists with stiffened perforations, leading to conservative or uncertain design. This study develops and validates a three-dimensional nonlinear finite-element model for CFS I-joists with lip-stiffened web openings. The model incorporates material and geometric nonlinearities, initial imperfections, and post-buckling behavior. It is validated against full-scale experimental results, including ten laboratory twin-joist floor assemblies and four field floor systems. The validated model is then used in a parametric study considering opening depth ratios, d_ℎ/ℎ, from 0.2 to 0.6, clear spacing from 0.5 h to 3.0 h, sheathing type, bracing configuration, floor-system width, and the presence of one to five openings along the joist length. Results show that lip-stiffened openings have only a minor effect on flexural strength, with an average reduction of approximately 3% compared to joists without openings, and a negligible impact on vibration frequency, even with multiple openings. Structural sheathing and bracing significantly enhance stiffness and delay lateral-torsional buckling. Elastic buckling and nonlinear post-buckling analyses further indicate that limiting the opening depth and clear spacing to 0.5 h provides efficient configurations for laterally braced joists. Based on the finite-element database, a new design equation is proposed that provides safe and more economical flexural-strength predictions than the Direct Strength Method for laterally braced CFS I-joists with lip-stiffened web openings.