Flexural behavior and design of cold-formed steel gapped back-to-back built-up beams with discontinuous connecting plates

This paper investigates the flexural behavior of a new type of cold-formed steel gapped back-to-back built-up beams, which are joined using discontinuous connecting plates and bolts under four-point bending. A finite element model was developed and validated against experimental results from the literature, focusing on ultimate moments, moment-curvature curves, and failure modes. Following successful validation, the model was used for an extensive parametric study to examine the impact of key parameters on the flexural behavior of the built-up beams. These parameters include bolt arrangement on each connecting plate, the ratio of connecting plate spacing to web depth, cross-sectional dimensions, the gap between the built-up channels, and beam length. The primary failure mode observed was lateral torsional buckling. The ultimate strengths obtained from the finite element analysis were compared with the nominal strengths predicted by the Direct Strength Method (DSM) specified in the American Specification. It was found that the DSM predictions were generally conservative. Consequently, a modified DSM equation was proposed. Comparisons with numerical results demonstrated that the modified DSM equation provided more accurate strength predictions than the current DSM. Reliability analysis confirmed the dependability of the modified DSM predictions, leading to the recommendation of the modified DSM for designing cold-formed steel gapped back-to-back built-up beams joined via discontinuous connecting plates and bolts.