Axial Compression Behavior of Circular Hollow CFST Composite Long Columns

Overview of the Study

This paper by Ren Qingxin and co-authors from Shenyang Jianzhu University investigates the axial compression performance of circular hollow concrete-filled steel tube (CHCCFST) composite long columns. The research combines finite element analysis (FEA) with experimental validation to establish a simplified formula for axial compression capacity. Supported by multiple funding sources including the National Natural Science Foundation (51808351), the Liaoning Xingliao Talent Program (XLYC1902027), and the Liaoning Key R&D Program (2020JH2/10300110), the paper was published in the Journal of Shenyang Jianzhu University (Natural Science), 2022, Volume 38, Issue 3, pages 410–417.

Core Technical Findings

The study establishes that concrete compressive strength (f_cu) and slenderness ratio (λ) are the two dominant parameters governing the axial compression performance of CHCCFST composite long columns. Key quantitative results include:

The simplified calculation formula proposed by the authors, based on the stability coefficient φ from existing codes, shows good agreement with both FEA and experimental results.

Finite Element Modeling Approach

The FEA model was validated against existing experimental data before being used for parametric studies. The model captures the load-midspan deflection curve characteristics, including initial stiffness, yield point, peak load, and post-peak softening behavior. The hollow core geometry is a distinguishing feature of CHCCFST columns compared to conventional solid CFST columns, offering weight reduction while maintaining structural efficiency.

Key Modeling Considerations

• Concrete constitutive model: The confined concrete model must account for the interaction between the steel tube and the hollow concrete core, which differs from solid CFST due to reduced confinement effectiveness at the hollow interface.
• Steel tube material model: Elasto-plastic constitutive law with kinematic hardening is typically employed to capture cyclic behavior and Bauschinger effects.
• Mesh convergence: The hollow geometry requires careful mesh density control, particularly at the inner wall of the steel tube where stress concentrations develop.
• Geometric nonlinearity: For long columns, geometric nonlinearity (P-Δ effects) is essential to capture the true buckling behavior.

Engineering Practice Integration

The CHCCFST composite long column concept is particularly relevant for high-rise and long-span structures where weight reduction is a priority. The hollow core reduces self-weight while the steel tube provides confinement and shear transfer. In composite column construction, the assembly of the hollow CFST component with the adjacent solid CFST or reinforced concrete segment requires careful attention to the interface connection.

From a steel pipe manufacturing perspective, the inner surface quality of the steel tube used for CHCCFST columns is critical. Residual stress from the forming process, surface oxide scale, and dimensional tolerance all affect the bond between the steel tube and the concrete. For circular tubes, the ERW or HFW welding process must produce a tube with uniform wall thickness and smooth internal surface to ensure consistent concrete confinement.

Design Implications

• Slenderness control: The significant capacity gain from reducing slenderness ratio (31.5% for λ reduction from 36 to 30) suggests that in tall building applications, optimizing column length through intermediate bracing or floor system integration can yield substantial structural economy.
• Concrete grade selection: The nonlinear relationship between concrete strength and capacity (42.4% gain for C60 vs. 62.8% for C80) indicates diminishing returns at higher grades, but also highlights the potential for ultra-high-strength concrete in extreme applications.
• Composite interface design: The connection between the hollow and solid CFST segments must be designed to transfer both axial force and bending moment. Bolted or welded steel sleeve connections are common solutions.

Reflections

This study makes a solid contribution to the understanding of CHCCFST composite columns, which represent an emerging structural system. The simplified formula based on the stability coefficient φ is practical for design use. However, the study focuses primarily on axial compression; in real structures, these columns will inevitably experience eccentric loading and biaxial bending. Future research should extend to eccentric compression and seismic performance. Additionally, the long-term behavior under sustained loads, including creep and shrinkage effects, remains unexplored and is critical for serviceability assessment.

The findings reinforce that hollow CFST composite columns offer a viable path for lightweight, high-capacity structural systems, provided that the parametric sensitivity identified in this study is properly accounted for in design.