Compressive Test Analysis of Square Steel Tube Hollow Sandwich Concrete Columns

Literature Overview and Context

The experimental study by Wang Xiannong, Zhang Fubin, and Zhou Xiaohui from Hebei Engineering University, published in 2013, investigates a novel composite column configuration known as the hollow sandwich steel tube concrete column (CFDST). This configuration features a square steel tube with a hollow core, where concrete is placed in the space between the outer steel tube and an inner steel tube, creating a sandwich structure. The research was supported by the Hebei Provincial Construction Fund and represents an innovative approach to enhancing the axial compressive capacity and flexural stiffness of composite columns.

Experimental Program and Methodology

Four CFDST square steel tube hollow sandwich concrete columns were tested under axial compression to evaluate their load-bearing performance, stiffness improvement, and failure characteristics. The test parameters included variations in steel tube dimensions, concrete strength, and the geometry of the hollow core. The primary objectives were to determine the axial compressive bearing capacity, measure the improvement in flexural stiffness relative to conventional steel tube concrete columns, and identify the dominant failure modes.

The specimens were instrumented with strain gauges and displacement transducers to capture the full load-deformation response. The loading was applied through spherical seat bearings to minimize eccentricity effects, and the tests were conducted under displacement control to capture the post-peak behavior.

Key Findings and Technical Analysis

The experimental results demonstrated that, within the tested parameter range, both the axial compressive bearing capacity and the flexural stiffness of the CFDST columns were effectively improved compared to conventional SRC columns of similar outer dimensions. The sandwich configuration leverages the composite action between the outer and inner steel tubes with the intervening concrete layer, creating a structure that benefits from both the confinement effect on the concrete and the structural efficiency of the hollow core geometry.

The failure modes observed included local buckling of the outer tube walls, concrete crushing in the confined layer, and in some cases, shear failure at the interface between the concrete and the inner tube. The researchers developed a bearing capacity calculation method that showed good agreement with the experimental results, providing a practical design tool for the CFDST configuration.

From a steel pipe fabrication standpoint, the CFDST configuration requires precise manufacturing of both the outer and inner square tubes, with tight dimensional tolerances to ensure uniform concrete layer thickness. The welding of the inner tube to the outer tube at the column ends — typically through end plates or direct weld connections — must be executed with high-quality procedures to ensure load transfer continuity. The hollow core also presents challenges for concrete placement and compaction, requiring carefully designed access openings and vibration protocols.

Engineering Practice Implications

For steel pipe suppliers, the CFDST configuration demands higher dimensional accuracy in square tube production, particularly regarding wall thickness uniformity and corner radius consistency. The inner tube, often of smaller dimensions, may require specialized forming equipment. Welding the end connections of both tubes to the column base and cap plates involves multi-layer multi-pass welding procedures that must be qualified to ensure full penetration and absence of internal defects such as lack of fusion or porosity.

The hollow core geometry also has implications for corrosion protection, as the enclosed space between the tubes may trap moisture and accelerate corrosion of the inner tube surface. Proper drainage provisions and corrosion-inhibitive coatings on the inner tube should be specified in the fabrication drawings.

Study Insights and Reflections

The CFDST configuration represents a creative approach to composite column design, but its practical adoption depends on the availability of cost-effective fabrication methods and the development of standardized design provisions. The experimental program, while providing valuable initial data, involved only four specimens, which limits the statistical robustness of the conclusions. A more comprehensive parametric study covering a wider range of geometric and material parameters would be beneficial before the configuration is widely adopted in practice. Nevertheless, the concept of using hollow sandwich structures to enhance composite column performance is promising and aligns with broader trends in lightweight and high-efficiency structural design.