Zhuojin Pipe Fitting Co., LtdAxial Compression Working Mechanism of Circular Hollow Sandwich Steel Tube Concrete Columns
Literature Overview
The paper by Huang Hong, Tao Zhong, and Han Linhai investigates the axial compression behavior of a novel composite column configuration: the circular hollow sandwich steel tube concrete (HSSCF) column. Published in Industrial Construction in 2006, this study employs ABAQUS finite element software to model the full load-deformation response of HSSCF columns under axial compression and analyzes the load-sharing mechanism between the inner steel tube, outer steel tube, and the concrete core sandwiched between them. The research was supported by the National Science Fund for Distinguished Young Scholars and the Fujian Province High-Level Talent Research Startup Fund, underscoring the significance of this research direction in the Chinese structural engineering community.
Structural Configuration and Design Rationale
The HSSCF column configuration consists of concentric inner and outer steel tubes with concrete placed in the annular space between them. This configuration differs from conventional CFST columns, where concrete fills the entire interior of a single steel tube, and from conventional hollow steel tube columns, which contain no concrete. The design rationale for the HSSCF configuration includes several practical considerations:
| Design Feature | Functional Benefit |
|---|---|
| Inner steel tube | Provides a formwork for concrete placement; serves as a structural member |
| Outer steel tube | Provides external confinement for concrete; resists external loads |
| Annular concrete fill | Provides confinement interaction with both tubes; adds stiffness and strength |
| Hollow core | Reduces self-weight; provides access for utilities or inspection |
The hollow core in the center of the column serves multiple purposes: it reduces the overall weight of the column without a proportional loss of load-bearing capacity, provides a channel for electrical conduits or piping, and allows for future inspection of the internal condition of the column. From a steel tube fabrication standpoint, the configuration also offers a practical advantage: the inner tube can serve as a formwork during concrete placement, eliminating the need for separate formwork construction.
Finite Element Modeling and Validation
The researchers used ABAQUS to develop a three-dimensional finite element model of the HSSCF column. The model was validated against experimental data, and the agreement between the calculated and measured load-deformation curves was reported to be good. The finite element model likely incorporated several key material models:
- Concrete: A plastic-damage model or Drucker-Prager model with confinement-dependent strength parameters, reflecting the triaxial compression state of the concrete between the two steel tubes.
- Steel tubes: A von Mises yield criterion with isotropic hardening, capturing the elastic-plastic behavior of the steel up to and beyond the yield point.
- Interface: Contact elements or embedded region techniques to model the interaction between the concrete and the steel tubes, including the possibility of debonding under large deformations.
The validation of the finite element model against experimental data is a critical step. In composite column research, the accuracy of the model depends heavily on the ability to capture the confinement interaction between steel and concrete. The fact that the calculated curves matched the experimental results well suggests that the chosen material models and interface conditions were appropriate for the HSSCF configuration.
Load-Sharing Mechanism Analysis
The core contribution of this study is the analysis of how the total axial load is distributed among the three components (inner tube, outer tube, and concrete) throughout the entire loading process. This analysis reveals the progressive nature of the load transfer:
- Elastic stage: In the initial elastic phase, the load is distributed among the three components in proportion to their respective axial stiffnesses. The concrete, having a lower modulus of elasticity than steel, carries a proportionally smaller share of the load.
- Yielding of steel tubes: As the load increases, the steel tubes reach their yield stress first. Beyond this point, the load transfer rate to the steel tubes decreases, and a progressively larger share of the additional load is carried by the concrete.
- Confinement activation: As the steel tubes undergo lateral expansion under axial compression, they exert a confining pressure on the concrete. This confinement increases the compressive strength of the concrete beyond its unconfined strength, creating a positive feedback loop: higher axial load leads to greater lateral expansion of the steel tubes, which increases the confinement pressure on the concrete, which in turn increases the load-carrying capacity of the concrete.
- Post-peak behavior: After the peak load is reached, the load-carrying capacity of the HSSCF column degrades gradually. The steel tubes continue to provide confinement even after yielding, and the concrete continues to carry a significant portion of the load, resulting in a ductile post-peak response.
The study further examined the influence of three key parameters on the interaction forces:
| Parameter | Effect on Interaction Force |
|---|---|
| Hollow ratio (core diameter / outer diameter) | Higher hollow ratio reduces the concrete volume and thus reduces the interaction force |
| Nominal steel ratio | Higher steel ratio increases the confinement pressure on the concrete |
| Material strength | Higher concrete strength increases the confinement interaction; higher steel yield strength increases the load-carrying capacity of the tubes |
Welding and Fabrication Considerations
From a fabrication standpoint, the HSSCF column presents several welding challenges that must be addressed in the design and construction phase:
- Inner tube welding: The inner steel tube must be fabricated and welded with precision to ensure a smooth inner surface, as any protrusions or weld beads on the inner surface would create stress concentrations in the concrete and could lead to local cracking.
- Outer tube welding: The outer steel tube is typically fabricated from plate or rolled tube segments, and the weld quality must meet the requirements of the applicable standard (e.g., GB/T 150, ASME BPV VIII, or EN 10216 depending on the application).
- Connection between inner and outer tubes: The relative position of the inner and outer tubes must be maintained during fabrication and erection. This is typically achieved using spacer rings or guide frames welded at regular intervals along the column length. The welding of these spacers introduces local stress concentrations that must be evaluated.
- Concrete placement access: The annular space between the tubes must be accessible for concrete placement and vibration. This may require the use of tremie methods or specialized pumping techniques, and the welding of access openings must be carefully planned to avoid compromising the structural continuity of the tubes.
Engineering Practice Integration
The HSSCF column concept has found application in several high-rise and industrial structures in China. The hollow core provides a practical advantage in buildings where mechanical and electrical services need to pass through the column core. However, the configuration also introduces additional complexity in terms of fabrication, quality control, and inspection.
From a quality control perspective, the HSSCF column requires several additional inspection steps compared to a conventional CFST column:
- Dimensional inspection of the annular gap: The gap between the inner and outer tubes must be uniform within tolerance to ensure even concrete fill and consistent confinement.
- Weld inspection of spacer connections: The spacer welds must be inspected to ensure they are sound and do not have defects that could lead to premature failure.
- Concrete fill verification: After concrete placement, the degree of fill in the annular space must be verified, typically using ultrasonic testing or gamma-ray densitometry.
- Post-weld inspection of the outer tube: The outer tube welds must be inspected after fabrication to ensure they meet the required quality level, as these welds are subject to the combined effects of axial compression and the lateral pressure from the confined concrete.
Study Insights and Future Directions
This study provides a solid foundation for understanding the behavior of HSSCF columns under axial compression. The key insight is that the hollow sandwich configuration offers a practical compromise between the high load capacity of conventional CFST columns and the weight and serviceability advantages of hollow tubes. The finite element model developed in this study can be extended to analyze the behavior of HSSCF columns under combined loading (axial force plus bending moment), which is more representative of actual structural conditions. Future research should also address the cyclic behavior of HSSCF columns under seismic loading, as this is critical for their application in earthquake-prone regions. The welding engineer must ensure that the fabrication details are compatible with the expected ductile behavior of the column under seismic loading, particularly at the weld joints of the steel tubes.