Mechanical Properties of Circular Hollow Sandwich Concrete-Filled Steel Tubes Under Axial and Eccentric Compression

Literature Overview

This study by Tao Zhong, Han Linhai, and Huang Hong (Fuzhou University, 2004) investigates the mechanical behavior of circular hollow sandwich concrete-filled steel tube (CFST) columns under both axial and eccentric compression. Funded by the Fujian Provincial Key Science and Technology Program (2002H007) and the Fujian Provincial Department of Construction, the research was published in the China Civil Engineering Journal, Vol. 37, No. 10, pp. 41-51. The work addresses a novel composite column concept that incorporates a hollow sandwich layer between the outer steel tube and the inner concrete core, offering potential advantages in weight reduction, thermal insulation, and corrosion protection.

Concept and Test Program

The hollow sandwich CFST concept involves a concentric arrangement where an outer steel tube encloses a hollow annular layer, which in turn surrounds an inner concrete core. This configuration differs from conventional solid CFST by introducing an air gap or lightweight infill between the steel and concrete layers.

The diameter-thickness ratio (D/t) is a critical parameter governing the local buckling resistance of the steel tube, while the hollow ratio (ratio of hollow diameter to outer diameter) determines the relative contribution of the steel tube versus the concrete core to overall load-bearing capacity.

Constitutive Models and Numerical Analysis

The authors developed stress-strain relationship models for both the steel and core concrete components, accounting for the unique interaction effects in the hollow sandwich configuration. The numerical analysis employed iterative solutions to capture the full load-deformation response from initial loading through ultimate failure.

Key modeling features:

• Steel: Elastic-perfectly plastic or bilinear hardening model with appropriate yield stress
• Concrete: Confinement-dependent model considering the reduced confinement effectiveness due to the hollow layer
• Interface behavior: Slip or bond-slip model between steel tube and concrete core
• Failure criterion: Combination of steel yielding and concrete crushing

The numerical results showed good agreement with experimental load-deformation curves, validating the proposed constitutive models and providing confidence in the parametric analysis and design formula development.

Parametric Analysis and Design Formulas

The parametric study revealed the following trends:

Based on the parametric analysis, the authors proposed practical verification methods for the bearing capacity of both axially compressed and eccentrically loaded hollow sandwich CFST members. These formulas provide engineers with a straightforward design tool that accounts for the unique structural behavior of this composite system.

Steel Tube Manufacturing and Welding Considerations

From a steel pipe manufacturing perspective, the hollow sandwich CFST concept introduces several technical challenges:

1. Tube geometry precision: The concentricity and dimensional accuracy of the outer tube are critical, as eccentricity between the tube and concrete core would create unintended bending moments and reduce load capacity.
2. Welding of connection details: Any circumferential welds or longitudinal welds in the outer tube must maintain geometric precision to ensure uniform concrete placement and effective confinement.
3. Surface preparation: The inner surface of the outer tube must be suitable for concrete adhesion, which may require shot blasting, thermal spray coating, or mechanical profiling.
4. Corrosion protection: The hollow layer provides inherent corrosion protection for the inner concrete core, but the outer tube surface requires appropriate coating or weathering steel for environmental durability.

Comparison with Conventional CFST

Engineering Practice Implications

The hollow sandwich CFST concept offers a viable alternative for applications where weight reduction, thermal performance, or corrosion protection are prioritized over maximum load capacity. Potential applications include:

• High-rise buildings in coastal or marine environments where corrosion is a concern
• Structures requiring thermal insulation in extreme climates
• Bridges and infrastructure where lightweight construction is advantageous
• Retrofitting applications where existing concrete cores can be enclosed with new outer tubes

For steel pipe suppliers, the hollow sandwich CFST concept creates market opportunities for precision-manufactured steel tubes with tight dimensional tolerances and high surface quality. Tubes should be manufactured per GB/T 8163 or API 5L with additional requirements for inner surface finish and concentricity.

Key Reflections

The study demonstrates that the hollow sandwich concept, while reducing ultimate capacity compared to solid CFST, provides meaningful benefits in terms of weight efficiency and environmental protection. The practical design formulas developed by the authors enable engineers to adopt this concept with confidence. From a manufacturing standpoint, the key challenge is maintaining geometric precision throughout the tube length, which requires advanced rolling or welding technology with tight process control.

The parametric findings regarding the hollow ratio provide guidance for optimizing the balance between capacity, weight, and durability. A hollow ratio of 0.4 to 0.6 appears to offer a reasonable compromise for most applications, though site-specific conditions may require different ratios. The study contributes to the diversification of composite column systems and expands the design toolbox available to structural engineers.