Finite Element Analysis of Ultimate Bearing Capacity of Axially Compressed Concrete-Filled Steel Tube Members

Literature Overview

This paper by Xu Xing, Cheng Xiaodong, and Ling Daosheng (2002), published in the Chinese Journal of Theoretical and Applied Mechanics, presents a finite element study on the ultimate bearing capacity of concrete-filled steel tube (CFST) columns under axial compression. The authors employ a three-dimensional virtual layered element method to analyze both the stability-bearing capacity and the material-bearing capacity of CFST members, and further determine the boundary between instability failure and material failure. The results are validated against experimental data and show good agreement. This study is foundational for understanding the dual failure mechanisms in CFST columns, which are widely used in high-rise buildings, bridges, and offshore platforms.

Theoretical Framework and Methodology

The key innovation in this study is the application of the three-dimensional virtual layered element method to CFST members. This method allows for the simulation of the interaction between the steel tube and the concrete core by treating the composite cross-section as a layered assembly, where each layer can independently deform while maintaining interface compatibility. This approach captures the confinement effect of the steel tube on the concrete and the constraining effect of the concrete on the steel tube buckling.

Analysis Aspect	Method	Key Output
Stability bearing capacity	Eigenvalue buckling analysis with virtual layers	First-order buckling mode and critical load
Material bearing capacity	Nonlinear material model with confinement effect	Ultimate axial load at material failure
Failure mode boundary	Comparison of stability and material capacity	Critical slenderness ratio
Buckling mode	First-order eigenmode simulation	Lateral deformation pattern

The stability bearing capacity represents the load at which the column becomes unstable (buckles) before the materials reach their individual strength limits. The material bearing capacity represents the load at which the steel tube yields and the concrete crushes under full confinement. The ultimate bearing capacity of the column is the lower of these two values.

Key Technical Results

The study establishes the following relationships for CFST columns:

Parameter	Effect on Stability Capacity	Effect on Material Capacity
Slenderness ratio (λ)	Strongly reduces stability capacity	No direct effect
Steel tube diameter-to-thickness ratio (D/t)	Reduces stability capacity due to local buckling	Slight reduction due to reduced confinement efficiency
Concrete strength (f_c)	No effect on stability capacity	Increases material capacity
Steel yield strength (f_y)	Increases stability capacity	Increases material capacity
Concrete fill ratio	Increases stability capacity	Increases material capacity

The boundary between instability failure and material failure is characterized by a critical slenderness ratio. Below this critical value, the column fails by material crushing (the steel tube yields and the concrete core is confined and crushed). Above this critical value, the column fails by global buckling before the materials reach their full capacity. The first-order buckling mode is simulated and shows a half-sine lateral deformation pattern, consistent with Euler buckling theory but modified by the composite action of the steel tube and concrete core.

Engineering Design Implications

For practical CFST column design, the following guidelines emerge from this study:

Slenderness ratio control: The slenderness ratio should be kept below the critical value to ensure that the column achieves its full material capacity through confinement.
Local buckling prevention: The steel tube diameter-to-thickness ratio should comply with code limits (typically D/t ≤ 50 for Q235 steel per GB 50017) to prevent local buckling of the tube wall.
Concrete confinement design: The concrete strength should be selected to complement the steel tube strength, ensuring that both materials contribute optimally to the composite action.
End condition consideration: The effective length factor (K) significantly influences the stability capacity and must be accurately determined based on the actual boundary conditions.

The study also highlights the importance of considering both stability and material failure modes in the design process. In many practical cases, especially for stocky columns with low slenderness ratios, the material capacity governs, and the confinement effect of the steel tube significantly enhances the concrete strength beyond its unconfined value.

Study Reflections

This paper provides a rigorous analytical framework for understanding CFST column behavior. The virtual layered element method is an effective approach for modeling the composite action without requiring extremely fine mesh refinement at the steel-concrete interface. However, the study is limited to axially compressed members, and the effects of eccentric loading, which are common in practical frames, are not addressed. Additionally, the study does not consider the influence of loading rate or strain rate effects, which may be relevant in seismic applications.

Conclusion and Outlook

The finite element analysis presented in this paper provides valuable insights into the dual failure mechanisms of CFST columns and establishes clear design guidelines for maximizing the composite action between the steel tube and concrete core. Engineers designing CFST columns should pay close attention to the slenderness ratio to ensure material capacity governs the design, and should verify that local buckling of the steel tube is prevented through appropriate diameter-to-thickness ratio control. The virtual layered element method offers a practical computational approach for analyzing complex CFST members, and its principles can be extended to more realistic loading conditions and boundary configurations in subsequent research.