Working Mechanism of Circular Hollow Sandwich Steel Tube Concrete Members Under Combined Compression and Torsion

Literature Overview

This study by Huang Hong, Fan Zhijie, and Chen Mengcheng (2013), published in the Journal of Guangxi University (Natural Science Edition), investigates the working mechanism of circular hollow sandwich steel tube concrete (SHS-CFST) members under combined axial compression and torsional loading. The research is conducted at East China Jiaotong University and is supported by the National Natural Science Foundation of China (Grants 51008122 and 50968006), the Jiangxi Provincial Department of Education (Grant GJJ10709), and the East China Jiaotong University Graduate Innovation Fund (Grant YC2011-X003).

Structural Configuration and Modeling Approach

The circular hollow sandwich steel tube concrete member consists of an outer steel tube, an inner steel tube, and a concrete layer sandwiched between them. This configuration provides enhanced confinement of the concrete core compared to conventional single-tube CFST members and offers potential advantages in terms of strength-to-weight ratio and damage tolerance.

The finite element modeling approach was first validated against existing experimental data for solid circular steel tube concrete members under combined compression and torsion. After validation, the same methodology was applied to the hollow sandwich configuration. The ABAQUS finite element model includes:

Model Component	Element Type	Material Model
Outer steel tube	Shell/Solid	Elastic-plastic (von Mises)
Inner steel tube	Shell/Solid	Elastic-plastic (von Mises)
Sandwich concrete	Solid	Confinement model (concrete damage plasticity)
Steel-concrete interface	Contact	Frictional contact with penalty method

Torsional Behavior and Three-Stage Response

The torque-angle relationship curves for the circular hollow sandwich steel tube concrete members under combined compression and torsion exhibit three distinct stages:

Elastic stage: Linear relationship between torque and angle of twist, with all components (outer tube, inner tube, and concrete) deforming elastically. The stress distribution is uniform across the cross-section.
Elastic-plastic stage: Yielding initiates at the outer surface of the outer steel tube, where the torsional shear stress is maximum. The yielding propagates inward as the torque increases. The concrete core begins to experience shear stresses transmitted through the steel-concrete interface.
Plastic hardening stage: Significant plastic deformation develops in both steel tubes, with strain hardening providing continued load resistance. The concrete core contributes to torsional resistance through shear transfer across the steel-concrete interface.

Axial Compression Ratio Effect

A key finding of this research is the non-monotonic effect of axial compression ratio on the ultimate torsional capacity:

Axial Compression Ratio	Effect on Ultimate Torque	Mechanism
Low (0 to moderate)	Increases	Confinement effect enhances concrete shear resistance; compressive stress delays yielding
High (moderate to high)	Decreases	Compressive stress accelerates shear yielding; reduced ductility; premature concrete crushing

This dual behavior has important implications for structural design. At low axial compression ratios, the confinement provided by the steel tubes enhances the concrete's contribution to torsional resistance. However, at high axial compression ratios, the compressive stress state reduces the material's capacity to resist shear deformation, leading to earlier failure.

Interaction Analysis Between Components

The finite element analysis provides detailed information on the interaction forces between the outer tube, inner tube, and concrete core during the loading process. Key observations include:

The outer steel tube carries the majority of the torsional moment throughout the loading process, particularly in the elastic and early plastic stages.
The inner steel tube contributes progressively more to torsional resistance as the outer tube yields and the deformation increases.
The concrete core transfers shear stresses to both steel tubes through the steel-concrete interface, with the frictional contact model accurately capturing the slip behavior at higher deformations.
The stress distribution cloud maps show that the maximum shear stress occurs at the outer surface of the outer tube, while the maximum compressive stress occurs at the inner surface of the inner tube.

Engineering Implications and Welding Considerations

For the fabrication of circular hollow sandwich steel tube concrete members, the following welding considerations are critical:

The circumferential welds joining the outer and inner tube end plates require full penetration with complete UT inspection, as these welds are subject to complex multi-axial stress states under combined compression and torsion.
The longitudinal seam welds of both tubes must maintain consistent quality throughout their length, as any weld defect can initiate cracking under torsional loading.
The welding sequence should be planned to minimize residual stresses that could interact adversely with the applied torsional moments.
For automated welding of the circumferential joints, the absence of internal supports (as recommended in Topic 2 research) is particularly beneficial for this sandwich configuration, where access to the inner tube weld seams would be restricted by any internal structure.

The research demonstrates that circular hollow sandwich steel tube concrete members offer enhanced structural performance under combined loading conditions, with the sandwich configuration providing superior confinement and damage tolerance compared to conventional single-tube designs. This makes them suitable for applications such as offshore platform columns, bridge piers, and industrial structures subject to complex loading combinations including seismic forces and wind-induced torsion.