Development and experimental verification of a Eurocode-based design method for local buckling for cold-formed C-sections encased in ultra-lightweight concrete under bending-compression interaction

Recently, an advancement in cold-formed steel (CFS) structures involving encasing sections in polystyrene aggregate concrete (PAC) has been presented. This new building system uses the strengths of both materials, with PAC providing heat insulation and fire resistance while also serving as continuous bracing thus enhancing the mechanical performance. This bracing function significantly enhances stability against buckling failures. This study presents an advanced design methodology for PAC-encased CFS elements, developed based on the Eurocode effective width method. The proposed framework builds upon previous research efforts in this field, integrating the bracing effect of the encasing material through analytical and numerical modelling. The PAC is modelled as an elastic half-space, analogous to sandwich beam theory, to account for its stabilizing influence. This study presents closed-form equations to determine the local critical stress of encased CFS sections under axial load and bending interaction. The predictive accuracy of the new design was assessed using 3D FEM and experimental results of the full CFS cross-section. The results showed a good correlation between calculated and FEM results, highlighting the reliability of the design procedure. The overall trend of the calculated results demonstrates that the proposed design equations can provide safe predictions when compared to the Eurocode linear interaction curve using buckling curve “d”. A new design method is proposed for cold-formed steel (CFS) sections encased in polystyrene aggregate concrete (PAC), incorporating Eurocode-based effective width principles. The PAC encasing material is modelled as an elastic half-space, acting as continuous lateral bracing that improves stability and local buckling resistance. Closed-form equations are derived to predict local buckling stress under axial-bending conditions for PAC-encased CFS members. The design prediction showed that using the buckling curve “d” offers more accurate and reliable strength estimations.