Behaviour and design of web-perforated cfs built-up box beams formed by plain channel sections

This study investigates the local buckling behaviour and design of web-perforated cold-formed steel (CFS) built-up box section beams, assembled from two plain channel sections connected by self-tapping screws. Twelve four-point bending tests were conducted to assess the ultimate capacity and failure modes, with local buckling in web and flange regions identified as the predominant failure mechanism. Then, validated finite element (FE) models were developed to replicate the test results and used to conduct parametric studies to examine the effects of section height-to-width ratio, plate thickness, web opening ratio, and screw spacing. Results indicated that increasing height-to-width ratios and plate thickness significantly enhanced bending capacity, whereas larger web perforations reduced strength and reduced screw spacing improved it. Finally, a modified Direct Strength Method (DSM) approach, incorporating reduction factors for opening ratio and screw spacing, was proposed to improve prediction accuracy and reliability, following a conservative assessment of the applicability of the current Effective Width Method (EWM) and DSM. This study provides valuable insights into the local buckling behaviour, improves design guidance for web-perforated CFS built-up beams, and identifies limitations and future research directions.