Axial Compression Bearing Capacity Calculation of Composite Steel Tube Concrete Based on Unified Theory

Literature Overview and Theoretical Foundation

This paper by Zhang Yufen, Zhao Junhai, and Li Xiaowei from Chang'an University, published in the Journal of Xi'an University of Architecture and Technology (Natural Science Edition) in 2009, extends the unified theory of steel tube concrete to composite (double-wall) steel tube concrete members. The research was supported by the Shaanxi Provincial Natural Science Foundation (SJ08E214) and Chang'an University Youth Fund (07Q01). The study addresses axial compression strength calculation for both solid composite and hollow composite steel tube concrete short columns by proposing an equivalent confinement coefficient.

The unified theory of steel tube concrete, originally developed to account for the confining effect of steel tubes on concrete, has been widely applied to single-wall CFST members. The extension to composite (double-tube) configurations introduces additional complexity due to the interaction between inner and outer steel tubes, the presence of a gap or intermediate layer, and the modified confinement mechanics.

Core Theoretical Framework

Equivalent Confinement Coefficient

The central innovation of this paper is the concept of the equivalent confinement coefficient for composite steel tube concrete. In single-wall CFST members, the confinement coefficient is defined as the ratio of the circumferential confining stress exerted by the steel tube on the concrete to the compressive strength of the concrete. For composite configurations, the authors propose an equivalent coefficient that accounts for:

• The combined confining action of both inner and outer steel tubes
• The geometric parameters of both tubes (diameter, wall thickness)
• The steel ratio of the composite section
• The hollow ratio for hollow composite members

Load-Bearing Capacity Formulas

The paper derives separate formulas for:

1. Solid composite steel tube concrete short columns
2. Hollow composite steel tube concrete short columns

Validation Against Experimental Data

The calculated results were compared with experimental data from published literature, demonstrating good agreement within specified ranges of steel ratio and hollow ratio. The authors establish that the unified theory remains valid for composite configurations provided the steel ratio and hollow ratio fall within appropriate bounds.

Technical Interpretation from a Pipe Manufacturing Perspective

From the standpoint of steel pipe fabrication and materials engineering, this research has several important implications. The composite steel tube configuration essentially represents a double-walled tube assembly, which raises questions about the manufacturing and assembly processes:

Manufacturing Considerations

• The inner and outer tubes must be concentrically aligned during assembly, requiring precision fabrication and assembly fixtures.
• The gap between inner and outer tubes, if any, may be filled with additional concrete, mortar, or left as a void depending on the design configuration.
• The weld seams of both tubes (typically ERW or SAW longitudinal welds) must be positioned to avoid stress concentration at critical load-bearing zones.

Material Compatibility

The confinement theory assumes elastic-plastic behavior of both steel tubes and the concrete core. The yield strength, ultimate tensile strength, and elastic modulus of the steel tubes must be known with sufficient accuracy for the theory to be applicable. For structural applications, typical steel grades include Q235, Q345, and Q390 per GB/T 1591, with corresponding ASTM A500 Gr. B or A572 Gr. 50 equivalents.

Confinement Effect and Tube Wall Thickness

The effectiveness of confinement depends on the slenderness of the tube walls. Thicker walls provide greater confining pressure but may reduce ductility. The width-thickness ratio (or diameter-thickness ratio for circular tubes) must remain below critical limits to prevent local buckling of the tube walls before the concrete reaches its confined strength. For circular tubes, this ratio is typically limited to approximately D/t ≤ 90 for carbon steel grades, with more restrictive limits for higher strength grades.

Standards and Design Code Context

The relevant standards for composite steel tube concrete design include:

The unified theory approach aligns with the confinement-based design philosophy adopted in JGJ 24, where the confined concrete strength is enhanced by a factor dependent on the steel tube's confining pressure. The extension to composite members represents an important theoretical advancement that could enable more efficient structural designs using double-wall tube configurations.

Engineering Practice Implications

The practical application of composite steel tube concrete members offers several advantages:

• Enhanced load-bearing capacity through the combined action of two steel tubes and confined concrete
• Improved ductility and energy dissipation through the layered confinement mechanism
• Potential for modular construction with pre-fabricated inner and outer tube assemblies
• Enhanced fire resistance and corrosion protection through the outer tube acting as a protective shell

However, several practical challenges must be addressed:

• The assembly and alignment of double-wall tubes requires specialized fabrication techniques and quality control procedures.
• The inspection and testing of the inner tube-concrete interface is more difficult compared to single-wall CFST members.
• The cost of additional steel and fabrication complexity must be justified by the performance benefits.

Key Questions and Reflections

The paper raises important questions about the applicability limits of the unified theory to composite configurations. Specifically, the interaction between the inner and outer tubes under differential deformation during loading is complex and may not be fully captured by the equivalent confinement coefficient approach. The assumption of uniform confinement pressure distribution may break down for members with high eccentricity or under combined loading conditions.

From a quality control perspective, the fabrication of composite steel tube members introduces additional inspection challenges. The concentricity of the inner and outer tubes, the integrity of the concrete fill, and the bond quality between the concrete and both tube surfaces are critical but difficult to verify non-destructively. Ultrasonic testing (UT) and radiographic testing (RT) may be employed, but the presence of the inner tube complicates signal interpretation.

The research also highlights the importance of material characterization for composite systems. The mechanical properties of both steel tubes and the concrete must be well-characterized, and the interaction between different material layers must be understood for reliable design predictions.

Study Insights and Outlook

This research represents a meaningful theoretical extension of the unified confinement theory to composite steel tube concrete members. The proposed equivalent confinement coefficient provides a practical design tool that bridges the gap between the well-established single-wall CFST theory and the emerging field of double-wall composite members. For practicing engineers, the key takeaway is that the unified theory framework remains applicable to composite configurations within defined parameter ranges, offering a rational basis for designing these advanced structural members. Future work should focus on validating the theory through full-scale testing, investigating the behavior under combined loading and cyclic loading conditions, and developing practical fabrication and inspection guidelines for composite tube members.