Axial Compression Mechanism and Bearing Capacity of Novel Double-Tube CFST Pier Columns

Literature Overview

The research by Zhang Yufen, Zhang Yan, and Jia Hongxin (2022), published in Industrial Construction, presents a novel composite pier column design that integrates both circular and rectangular steel tubes within a single structural element. The proposed design features a rectangular outer steel tube with two eccentrically placed circular inner steel tubes, creating a multi-level confinement system for the core concrete. The study combines theoretical derivation based on limit equilibrium theory with three-dimensional finite element analysis using ABAQUS, validated against experimental results.

Novel Structural Configuration

The proposed double-tube CFST pier column represents an innovative approach to maximizing the confinement effect on core concrete. The structural configuration consists of three distinct components:

A rectangular outer steel tube that provides primary confinement and structural integrity.
Two circular inner steel tubes (eccentrically placed) that provide secondary confinement to the core concrete.
Concrete filling the entire cross-section, including the space between the outer and inner tubes.

This configuration leverages the superior confinement efficiency of circular tubes (which provide uniform radial confinement) while maintaining the structural advantages of rectangular sections (which are more efficient for moment resistance in two orthogonal directions). The eccentric placement of the inner tubes is a deliberate design choice that optimizes the distribution of confinement pressure.

Theoretical Bearing Capacity Derivation

Based on limit equilibrium theory, the authors decomposed the composite column into three components for analysis:

Component	Analytical Approach	Key Assumptions
Rectangular outer steel tube	Accounts for differential deformation of long and short sides	Elastic-plastic behavior with strain hardening
Interlayer concrete (between outer and inner tubes)	Confined concrete model with triaxial stress state	Uniform confinement pressure from outer tube
Inner circular tube CFST	Standard confined concrete model	Uniform radial confinement from circular tube

The derived bearing capacity formula accounts for the different confinement effects provided by the rectangular outer tube and the circular inner tubes. The theoretical predictions were compared with finite element results, yielding an average ratio of 0.923 with a standard deviation of 0.046, indicating good agreement between theory and simulation.

Parametric Analysis and Key Findings

The finite element parametric study investigated the influence of several key parameters on the ultimate bearing capacity:

Parameter	Effect on Bearing Capacity	Optimal Range
Concrete compressive strength (f_c)	Increases capacity	Higher is better (within practical limits)
Steel tube yield strength (f_y)	Increases capacity	Higher is better (within practical limits)
Outer tube width-to-thickness ratio	Decreases capacity	Should be limited to prevent local buckling
Inner tube diameter (same steel ratio)	Increases capacity up to ~2/3 outer width	Optimal at approximately 2/3 of outer tube width
Inner tube diameter-to-thickness ratio	Moderate influence	Should be controlled for local buckling

A particularly noteworthy finding is that when the inner tube diameter reaches approximately two-thirds of the outer tube width, further increases in diameter provide diminishing returns on bearing capacity. This suggests an optimal geometric configuration that balances the confinement benefit of larger inner tubes against the reduced interlayer concrete volume.

Engineering Practice and Manufacturing Considerations

From a steel pipe manufacturing perspective, the fabrication of this novel double-tube CFST column involves several critical considerations:

The rectangular outer tube can be fabricated by HFW (high-frequency resistance welding) or LSAW (longitudinal submerged-arc welding), depending on the required dimensions and wall thickness.
The circular inner tubes are typically seamless or ERW welded, selected based on the required diameter and wall thickness.
The welding of inner tubes to the outer tube requires careful attention to weld quality, particularly at the through-thickness welds connecting the inner circular tubes to the inner surface of the rectangular outer tube.
The concrete pouring process must ensure complete filling of all cavities without voids, which is challenging in the complex geometry of the double-tube configuration.

The welding sequence is critical to minimize residual stress and distortion. A recommended approach involves:

Fabricate and inspect the rectangular outer tube first.
Fabricate and inspect the circular inner tubes.
Position the inner tubes within the outer tube at the specified eccentricity.
Weld the inner tubes to the outer tube using GTAW or FCAW, with careful control of interpass temperature to limit thermal distortion.
Perform non-destructive testing (RT or UT) on all welds, particularly the through-thickness joints.

Summary and Implications

This study presents a conceptually elegant solution to the challenge of maximizing concrete confinement in CFST columns by combining the geometric advantages of rectangular and circular sections. The theoretical framework based on limit equilibrium theory provides a practical design tool, validated by both finite element analysis and experimental results. The identification of the optimal inner tube diameter at approximately two-thirds of the outer tube width offers clear design guidance. For steel pipe manufacturers and welding engineers, the fabrication of such novel composite columns requires careful planning of welding sequences, rigorous quality control of through-thickness welds, and attention to the complex concrete pouring requirements. The research opens new possibilities for high-capacity pier column design in bridge and infrastructure engineering.