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

Literature Overview and Research Context

This paper by Huang Hong, Fan Zhijie, and Chen Mengcheng, published in the Journal of Guangxi University (Natural Science Edition) (2013, Vol. 38, No. 1, pp. 42-47), investigates the working mechanism of circular hollow sandwich steel tube concrete (SHSTC) members under combined compression and torsion loading. The research was supported by the National Natural Science Foundation of China (Grants No. 51008122 and 50968006), the Jiangxi Provincial Department of Education (GJJ10709), and the East China Jiaotong University Graduate Innovation Fund (YC2011-X003).

As a metallurgist and welding engineer, I find this research relevant to the design of tubular structures subjected to complex loading states, such as wind turbine towers, transmission line poles, and offshore platform legs. The sandwich construction—where two concentric steel tubes encase a concrete core—offers advantages in terms of corrosion protection, fire resistance, and structural efficiency. The study of torsional behavior is particularly important for structures subjected to wind or seismic torsional moments.

Core Technical Findings

The authors used ABAQUS finite element software to model both solid circular steel tube concrete (SSTC) members and hollow sandwich steel tube concrete (SHSTC) members under combined compression and torsion. The FEA methodology was validated against existing experimental data for SSTC members before being applied to SHSTC members.

The study reveals a non-monotonic relationship between the axial compression ratio and the ultimate torsional capacity. At low axial compression ratios, the compressive stress enhances the torsional capacity through confinement and increased friction between the concrete and the steel tubes. At high axial compression ratios, the concrete reaches a state of high compressive stress that reduces its ability to contribute to torsional resistance, and the ultimate torque decreases.

Interpretation of Technical Points

Three-Stage Torque-Rotation Behavior

The torque-rotation relationship exhibits three distinct stages:

1. Elastic stage: All components (inner tube, outer tube, concrete) behave elastically. The torque is proportional to the rotation angle, and the effective torsional rigidity is the sum of the individual contributions.
2. Elasto-plastic stage: The outer steel tube begins to yield at the outer fiber, while the inner tube and concrete remain elastic. The torsional rigidity decreases as the plastic zone spreads inward.
3. Plastic hardening stage: Both steel tubes have yielded, and the concrete is in a state of multiaxial stress. The torque continues to increase due to strain hardening of the stainless steel or high-strength steel tubes and the confinement effect of the concrete on the inner tube.

Axial Compression Ratio Effect

The axial compression ratio (n = N / N_u, where N is the applied axial load and N_u is the ultimate axial capacity) has a complex effect on torsional capacity:

• Low n (n < 0.3): The axial compression provides a beneficial confinement effect on the concrete, increasing the effective shear strength and thus the torsional capacity. The friction between the concrete and the steel tubes also contributes to torsional resistance.
• High n (n > 0.6): The concrete is already heavily compressed, and additional axial load does not significantly increase confinement. Instead, the high compressive stress reduces the concrete's ability to resist shear, and the ultimate torsional capacity decreases.

This non-monotonic behavior has important design implications. Structures subjected to combined compression and torsion should be designed to avoid operating in the high axial compression ratio regime if maximum torsional capacity is desired.

Sandwich Construction Advantages

The hollow sandwich configuration offers several advantages over a single solid steel tube:

• Improved torsional stiffness: The outer tube contributes significantly to torsional rigidity due to its larger radius, while the inner tube provides additional confinement to the concrete core.
• Enhanced fire resistance: The concrete core provides thermal insulation to the inner tube, and the air gap between the tubes can be filled with fire-resistant material.
• Corrosion protection: The outer tube protects the concrete and inner tube from environmental exposure, extending the service life.
• Efficient material use: The sandwich configuration allows for optimized material distribution, with steel where tensile stresses are high and concrete where compressive stresses are high.

Integration with Engineering Practice

Welding Considerations for Sandwich Tubes

The fabrication of hollow sandwich steel tube concrete members involves welding of both the inner and outer tubes, as well as the connection between the two tubes at the ends. Key welding considerations include:

• Outer tube longitudinal seam: Typically FCAW or SAW welding, with full-penetration groove preparation. The weld must achieve full fusion and be free of defects such as slag inclusion, porosity, and incomplete penetration.
• Inner tube longitudinal seam: Similar to the outer tube, but with additional challenges due to access limitations. GTAW or FCAW with automatic welding equipment is preferred.
• End ring welds: The connection between the inner and outer tubes at the column ends requires careful weld design. Fillet welds or groove welds must be used to transfer shear forces between the tubes.
• Heat input control: For stainless steel or high-strength steel tubes, heat input must be controlled to prevent excessive HAZ softening or distortion.

Quality Control and NDT

Design Implications for Combined Loading

For structures subjected to combined compression and torsion, the design must account for the interaction between the two load components. The following design checks are recommended:

• Axial capacity check: N ≤ N_u, where N_u is the ultimate axial capacity of the composite member.
• Torsional capacity check: T ≤ T_u(n), where T_u(n) is the ultimate torsional capacity at the given axial compression ratio n.
• Interaction check: (N/N_u)^2 + (T/T_u)^2 ≤ 1.0 (approximate interaction equation).
• Shear buckling check: For thin-walled tubes, the web panels between stiffeners must be checked for shear buckling under combined loading.

Key Questions and Reflections

The study focuses on the static behavior of SHSTC members under combined compression and torsion. The dynamic behavior under cyclic or impact loading is not addressed, which is a limitation for seismic design applications. The strain-hardening behavior of the steel tubes under torsional loading may provide advantages in energy dissipation, but this requires further experimental investigation.

Another open question is the effect of the air gap between the inner and outer tubes on the torsional behavior. The study assumes full composite action between the inner tube, concrete, and outer tube. In practice, the bond between the concrete and the inner tube may be imperfect, and the air gap may not be fully filled with concrete. This could reduce the effective torsional rigidity.

The long-term behavior under sustained loading is also not addressed. Creep and shrinkage of the concrete core may lead to differential deformation between the concrete and the steel tubes, potentially reducing the composite action over time.

Study Insights and Implications

This research provides valuable insights into the working mechanism of circular hollow sandwich steel tube concrete members under combined compression and torsion. The identification of the three-stage torque-rotation behavior and the non-monotonic effect of axial compression ratio on torsional capacity are important findings that should be incorporated into design methodologies. The FEA model, validated against experimental data, offers a reliable analytical tool for predicting the behavior of SHSTC members under complex loading states. For engineers involved in the design and fabrication of tubular composite structures, the key implications are: (1) the sandwich configuration provides enhanced torsional performance compared to single-tube systems; (2) the axial compression ratio should be optimized to maximize torsional capacity; and (3) rigorous quality control of welds and concrete is essential to ensure the assumed composite action is achieved in practice.