Load-Bearing Mechanism of Composite Steel Pipe Concrete Columns Under Axial Compression

Literature Overview

This paper, published in the Journal of Henan Polytechnical University (Natural Science Edition) (2014, Vol. 33, No. 4, pp. 516-520) by Wang Qinting, Luo Xulin, Zhang Chunlei, and Chang Xu from Henan Polytechnical University, investigates the load-bearing mechanism of composite steel pipe concrete (CSTC) columns featuring an external square steel tube with an internal circular steel tube configuration. The study employs a plastic damage model for concrete and compares the axial stress distribution in CSTC columns with conventional single-tube CFST columns under axial compression. Funded by the National Natural Science Foundation of China (grant 41172317) and the Henan Provincial Youth Backbone Teacher Research Project (2012GGJS-058), the research reveals that the dual-confinement effect significantly enhances the core concrete's axial stress capacity and ultimate load-bearing capacity.

Core Technical Points

Structural Configuration and Dual-Confinement Mechanism

The composite steel pipe concrete column studied features an external square steel tube enclosing an internal circular steel tube, with concrete filling both the annular space between the tubes (intermediate layer) and the interior of the circular tube (core). This configuration creates a unique dual-confinement mechanism:

• Inner tube confinement: The internal circular steel tube provides direct lateral confinement to the core concrete, generating radial confining pressure through Poisson's effect as the concrete expands laterally under axial compression.
• Outer tube confinement: The external square steel tube provides additional lateral support to the intermediate-layer concrete, which in turn transmits confinement pressure to the core concrete through the intermediate concrete layer.

The research reveals that the core concrete experiences significantly higher axial stress compared to conventional single-tube CFST columns due to this dual-confinement effect, while the intermediate-layer concrete shows axial stress values comparable to those in single-tube configurations.

Axial Stress Distribution Patterns

The plastic damage model employed for concrete analysis captures the progressive degradation of concrete stiffness under increasing compressive strain, which is essential for accurately representing the nonlinear stress-strain behavior in the confined concrete zones. The model parameters, including the damage variable evolution law and the confined concrete strength model, play critical roles in predicting the load-bearing capacity and deformation characteristics.

Parametric Influence on Mechanical Performance

The study examined the effects of steel tube wall thickness and concrete strength grade on the mechanical performance of CSTC columns:

• Steel tube wall thickness: Increasing the wall thickness of both the inner and outer steel tubes enhances the confining pressure capacity, leading to improved ductility and ultimate load-bearing capacity. The inner tube wall thickness has a more direct influence on core concrete confinement, while the outer tube wall thickness primarily affects the intermediate-layer concrete behavior.
• Concrete strength grade: Higher concrete strength grades contribute to increased initial stiffness and peak load capacity. However, the confinement effect becomes more significant at higher concrete strengths, as the confined concrete can sustain higher strains before failure, allowing more confining pressure to develop.

Welding and Fabrication Considerations

From a steel pipe and welding engineering perspective, the CSTC column configuration presents specific fabrication challenges:

1. Inner tube welding: The internal circular steel tube must be fabricated with high-quality welds, as weld defects in the inner tube directly affect the confinement capacity of the core concrete. GTAW (Gas Tungsten Arc Welding) or SAW (Submerged Arc Welding) processes are typically employed for inner tube fabrication.
2. Outer tube welding: The external square steel tube is commonly fabricated from rolled steel plates using SAW welding. Weld quality, particularly in the longitudinal seams, is critical for maintaining the structural integrity of the outer confinement system.
3. Tube-to-tube connection: The connection between the inner and outer steel tubes, if required, must be designed and fabricated to ensure proper load transfer and prevent premature separation under high confinement pressures.
4. Concrete placement: The annular space between the inner and outer tubes requires careful concrete placement to ensure full fill and adequate compaction. Insufficient compaction in the intermediate layer can compromise the confinement transmission mechanism.

Critical Reflection and Independent Analysis

The dual-confinement mechanism identified in this study represents an innovative approach to enhancing CFST column performance, but several engineering considerations warrant attention:

• Construction complexity: The CSTC configuration requires precise alignment of the inner tube within the outer tube, which adds complexity to fabrication and erection. Tolerances for concentricity and vertical alignment must be tightly controlled.
• Concrete placement challenges: Filling the annular space between two concentric tubes with concrete is inherently more challenging than filling a single tube. Honeycombing, voids, and insufficient compaction are potential risks that must be mitigated through proper vibrator placement and concrete mix design.
• Cost-benefit analysis: The enhanced load-bearing capacity must be evaluated against the increased material and fabrication costs. The CSTC configuration uses more steel and requires more complex fabrication, which may not be economically justified for all applications.
• Standardization gaps: At the time of publication, the CSTC configuration was not covered by existing design codes. The study's findings provide a basis for developing design provisions, but further experimental validation with larger-scale specimens and more comprehensive parametric studies are needed.

The finding that intermediate-layer concrete axial stress is comparable to single-tube CFST values is particularly interesting from a materials science perspective. It suggests that the intermediate concrete primarily functions as a load-transfer medium rather than a confined structural element, which has implications for the optimal thickness of the intermediate layer.

Reference Value and Outlook

This research provides valuable insights into the load-bearing mechanism of composite steel pipe concrete columns, demonstrating that dual-confinement configurations can significantly enhance structural performance. The findings contribute to the development of innovative structural systems that combine the advantages of steel and concrete in novel configurations. Future research should address fabrication optimization, cost-effectiveness analysis, and the development of code provisions for CSTC columns. The practical implementation of this concept requires close coordination between steel pipe fabricators, concrete contractors, and structural engineers to ensure that the theoretical benefits are realized in actual construction.