Compressive Test Analysis of Square Steel Tube Hollow Sandwich Concrete Columns

Literature Overview

The paper by Wang Xianong, Zhang Fubin, and Zhou Xiaohui, published in the Journal of Hebei Engineering University (Natural Science Edition) in 2013, presents experimental investigation of concrete-filled double-skin square steel tube (CFDST) columns. This composite structural system consists of an outer square steel tube, an inner square steel tube, and a concrete layer sandwiched between them, creating a hollow-core column that offers enhanced bending stiffness and compressive capacity compared to conventional single-skin CFST columns.

Structural Configuration and Test Parameters

The CFDST column system features a distinctive three-layer configuration:

Component	Function	Typical Material
Outer square steel tube	Primary compression member	Q235/Q345 carbon steel
Inner square steel tube	Core confinement and stiffness enhancement	Q235/Q345 carbon steel
Sandwich concrete layer	Confinement medium and stiffness contributor	C30–C50 concrete
Hollow core	Weight reduction and architectural flexibility	Empty or filled with non-structural material

Four CFDST square steel tube hollow sandwich concrete columns were tested under axial compression. The test parameters varied to investigate the influence of geometric proportions and material properties on structural performance.

Experimental Findings

The key findings from the axial compression tests are summarized as follows:

Load-bearing capacity enhancement: The CFDST columns demonstrated significantly higher axial compressive capacity compared to conventional CFST columns of equivalent outer dimensions, attributed to the increased steel cross-sectional area and the enhanced confinement effect from the double-skin configuration.
Bending stiffness improvement: The double-skin configuration provides substantially greater bending stiffness, which is critical for columns subjected to combined axial and bending loads. The increased moment of inertia from the hollow core geometry contributes to this improvement.
Failure modes: The observed failure patterns included:

Local buckling of the outer tube walls
Crushing of the sandwich concrete layer
Local buckling of the inner tube walls
Combined failure involving multiple mechanisms

Load-displacement behavior: The CFDST columns exhibited a more pronounced post-peak load-carrying capacity compared to single-skin CFST columns, indicating superior ductility and energy absorption characteristics.

Calculation Method and Validation

The authors proposed a calculation method for the bearing capacity of CFDST columns that considers:

The axial compression capacity of the outer steel tube
The axial compression capacity of the inner steel tube
The enhanced concrete strength due to the double-sided confinement
The interaction between the two steel tubes through the sandwich concrete

The calculated results showed good agreement with the experimental data, validating the proposed calculation method. The confinement effect on the sandwich concrete is modeled as a biaxial confinement condition, where the concrete is confined by both the inner and outer steel tubes.

Engineering Practice Considerations

From a steel pipe fabrication and construction perspective, the CFDST system presents several challenges:

Manufacturing complexity: The fabrication requires precise alignment of two concentric square steel tubes with a controlled concrete layer thickness. Tolerances on tube dimensions and wall thickness are critical.
Welding considerations: The connection between the inner and outer tubes must be designed to maintain the required spacing while providing structural continuity. Welding of the inner tube to the outer tube through the concrete layer is not feasible; therefore, mechanical spacers or welded rings are required.
Quality control: Non-destructive testing (NDT) of the sandwich concrete layer is challenging due to its confined position. Ultrasonic testing (UT) or ground-penetrating radar may be required to verify concrete quality and detect voids.
Concrete placement: The sandwich concrete must be carefully placed and vibrated to ensure full compaction between the two steel tubes. This may require specialized placement techniques such as pumped concrete with controlled slump or the use of micro-concrete.

Key Technical Parameters

Parameter	Influence on Performance	Design Consideration
Outer tube wall thickness	Primary load-bearing capacity	Must satisfy local buckling criteria
Inner tube wall thickness	Confinement enhancement and stiffness	Minimum thickness for stability
Sandwich concrete thickness	Confinement effectiveness and stiffness	Must be sufficient for vibration and bonding
Concrete strength grade	Confinement capacity and post-peak behavior	Higher grade improves confinement but reduces ductility
Steel grade	Yield capacity and ductility	Q345 preferred for high-strength applications

Study Insights and Implications

The CFDST column system represents an innovative approach to achieving high compressive capacity and bending stiffness in a compact cross-section. The experimental results demonstrate that the double-skin configuration provides meaningful improvements over conventional CFST columns, particularly in terms of post-peak ductility and stiffness. For steel pipe manufacturers, the demand for precisely fabricated square tubes with tight dimensional tolerances will increase as this system gains acceptance. The quality of the sandwich concrete layer—its density, strength, and bonding to both steel surfaces—is critical to the overall structural performance, and this presents a unique quality control challenge that requires innovative inspection methods.