Flexural Integrity Between the Individual Channels of Built-Up Cold-Formed Steel Beams

This paper investigates the influence of commonly used screw connections on the integrity of built-up beams formed by connecting two lipped channel sections in a back-to-back configuration. An experimental investigation on the behaviour of 6 built-up CFS beams connected by self-drilling screws is presented in this paper. The screw spacing was varied along the length as well as the depth of the beams. The test results indicate that the screw spacing has some influence on the capacity of the built-up beams. An increase in screw spacing leads to an undesirable separation of channels and thus has a negative impact on the composite action between the members. Finite element models were developed and after successful validation, a parametric study involving 124 models was carried out, to investigate the impact of larger longitudinal screw spacings on the integral behaviour of the beams. It was observed that the flexural capacity decreased by 11%, 14%, 17% and 19% when the longitudinal distance between the screws increased from 50 to 1050 mm for beams with web depths 150 mm, 200 mm, 250 mm and 300 mm respectively. It can be concluded that flexural capacity is directly proportional to the decrease in the screw spacing and increase in the number of screw rows. The results also indicate that the percentage decrease in strength is greater for beams with larger web depths. An idealized cross section assuming complete composite action was simulated for each web depth using FE modelling and the capacities of all the screw-connected “semi-rigid” beams were compared against it. The capacity of screw connected built-up beams with web depths 150 mm, 200 mm, 250 mm and 300 mm was found to be 0.71, 0.64, 0.61 and 0.59 times that of the idealized cross-section on average, respectively. The moment capacities of screw-connected beams were also compared against the theoretical values obtained from AISI S-100. It was observed that the design guidelines provided in AISI S-100 were unable to account for the reduction in the moment capacity due to the screw connection in built-up members. AISI S-100 is conservative by as much as 34%, 42%, 46% and 48% for beams with web depths 150 mm, 200 mm, 250 mm and 300 mm respectively.