The paper discusses the structural behaviour of concrete-filled steel tube columns (CFT) when applied to the top-down construction method as temporary internal supports for ceilings. Their ultimate capacity to take into account the actual boundary conditions of the column is essential for successful application in construction. The paper presents the full-scale in situ testing of four slender specimens with variable D/t ratios under concentric axial loading. The CFT columns were supported on the previously jacked concrete piles. In addition, detailed finite element numerical models in ABAQUS and PLAXIS computer programs were developed. The models include the nonlinear behaviour of materials and the nonlinear behaviour of soil. The soil–pile–column interaction and impact of the CFT column–pile connection stiffness on global column stability were considered. The numerical model was validated by comparison with the experimental results. In conclusion, the coefficient for the effective buckling length of the studied columns is proposed. Finally, the experimental results of the critical buckling forces were compared with widely used international design codes Eurocode 4-EC4, American standard-ACI and the Australian standard-AS.

The paper presents three-dimensional numerical models of short concrete-filled steel tubular circular columns that can successfully describe the column behaviour under axial compression. Several of the most commonly used material models for the steel part and a concrete portion of the column are evaluated in the models. In addition, the paper presents a new proposal for the extension of the Eurocode 2 stress-strain relation to make it suitable for describing the complex behaviour of concrete inside the steel tube. This model overcomes the current limitations of the Eurocode 2 design guide, referring to the limitation for the concrete curve in compression to 3.5%o strain. The ultimate axial column strength obtained by the proposed model is compared to the ultimate column capacity calculated by a simplified method provided in Eurocode 4. All presented numerical models are validated on a set of experiments from the literature and demonstrate good agreement. The comments about the accuracy of each model are provided, along with the identified limitations.

This paper focuses on the experimental investigation of the axial load capacity of CFT (concrete-filled steel tube) columns under actual construction conditions during building reconstruction. A total of four samples were loaded up to failure. The varied parameters were the column length and absence/presence of the concrete infill within the steel tube. Further, the analysis is extended to developing a numerical model in the finite element-based software ABAQUS version 6.9. This numerical model includes material and geometrical nonlinearities and was validated with the experimental results. The contribution of the concrete core to the column capacity and the concrete core confinement effect are discussed. Finally, the column capacity was calculated according to several design codes: the Eurocode 4 with and without considering the confinement effect, American specifications, Australian standards, the American Institute of Steel Construction, and the Architectural Institute of Japan. The Eurocode 4 considering the confinement effect provides the closest results to those obtained in the tests.

The paper analyses the post-fire axial capacity of recycled aggregate concrete-filled steel tube (RACFST) columns. Since existing design codes lack guidelines for calculating this capacity, the modified method from the second-generation of Eurocode 4 is applied and its applicability is confirmed for RACFST columns. For method evaluation, the results from previous experimental campaigns are compiled. Additionally, to obtain a better understanding of the advantages of the RACFST solution compared to conventional RC columns regarding sustainability, the assessment of post-fire capacity for two chosen sections is conducted. A fair comparison is made to analyze the post-fire capacities of unprotected RACFST and RC section.

Concrete-filled steel tubes incorporating recycled aggregate concrete represent a sustainable composite structural members in which the steel tube provides effective confinement and enhances column performance. Although the experimental database for such members has grown in recent years, robust high–fidelity numerical models remain limited. This study presents a computational framework that combines Bayesian optimisation with nonlinear FEM analysis in ABAQUS, automated through MATLAB–Python scripting. The approach optimises key parameters of the confined concrete stress–strain relationship while reproducing experimental axial load–displacement responses of stub columns in compression. The optimised parameters are then used in combination with the regression analysis to develop reliable nonlinear constitutive models for the concrete within steel tube. Additional comparison with two established confined concrete models shows good agreement in load–displacement behaviour and improved performance. The proposed framework is efficient and transferable.