Axial Compression Short Column Bearing Capacity of Circular Hollow Sandwich Concrete-Filled Steel Tube

Overview and Context

The paper by Wang Xianong, Gao Congchao, and Wang Nannan, published in the Journal of Hebei Engineering University (Natural Science Edition) in 2013, investigates the axial compression bearing capacity of circular hollow sandwich concrete-filled steel tube (CFST) short columns. This innovative structural form consists of an inner steel tube, an outer steel tube, and a concrete layer sandwiched between them, creating a three-layer composite member with enhanced confinement and load-bearing capacity. From a steel pipe fabrication and welding perspective, this configuration presents unique challenges related to the welding of two concentric tubes and the concrete placement between them.

Theoretical Framework: Double Shear Unified Strength Theory

The authors employ the double shear unified strength theory (also known as the twin-shear unified strength theory) to analyze the stress state of the core concrete and the steel tubes under axial compression. This theoretical framework is significant because it accounts for the effect of intermediate principal stress on material strength, which is particularly important for the triaxial stress state experienced by the sandwich concrete layer.

The key analytical steps include:

1. Analyzing the lateral compressive stress exerted by the core concrete on the inner and outer steel tubes under axial compression
2. Determining the horizontal bearing stress in the inner and outer steel tubes
3. Deriving the vertical ultimate bearing stress in the inner and outer steel tubes using the double shear unified strength theory
4. Combining the steel tube capacities with the core concrete capacity to derive a simplified formula for the total axial compression bearing capacity
5. Solving the formula numerically and comparing results with experimental data from existing literature

Fabrication Challenges for Sandwich CFST Members

The sandwich CFST configuration introduces several fabrication challenges that must be addressed to achieve the theoretical performance predicted by the analytical model:

Bearing Capacity Analysis

The double shear unified strength theory provides a more accurate prediction of concrete strength under triaxial stress conditions compared to traditional theories that only consider the maximum and minimum principal stresses. For the sandwich CFST configuration, the concrete layer experiences a complex stress state with:

• Axial compressive stress from the applied load
• Lateral compressive stress from the Poisson effect of the concrete itself
• Lateral compressive stress from the confinement of the inner and outer steel tubes

The confinement effect is particularly important because it can increase the concrete strength significantly beyond its unconfined compressive strength. The degree of confinement depends on:

1. The diameter-to-thickness ratio of the steel tubes
2. The yield strength of the steel tubes
3. The thickness of the concrete layer
4. The concrete compressive strength

The simplified formula derived in this paper provides a practical tool for engineers to estimate the bearing capacity of sandwich CFST columns, but its accuracy depends on the quality of the fabrication and the integrity of the concrete-steel bond.

Welding Considerations for Sandwich Configuration

The welding requirements for sandwich CFST members are more demanding than for conventional CFST members because of the concentric tube configuration:

1. Inner tube end welds: The inner tube must be welded to the end plates with full penetration to ensure load transfer. These welds are difficult to access and inspect, requiring advanced NDT methods such as phased array ultrasonic testing (PAUT) or magnetic flux leakage (MFL).
2. Outer tube end welds: The outer tube end welds are more accessible but still require careful qualification due to the thick section that may result from the combined thickness of the outer tube and end plate.
3. Heat input control: The welding of both tubes must be carefully sequenced to minimize distortion and residual stress. Welding the inner tube first and then the outer tube can help control overall distortion, but the heat input from the outer tube weld can affect the previously deposited inner tube weld.
4. Residual stress interaction: The residual stresses from the inner and outer tube welds interact in a complex manner, potentially creating high residual stress concentrations at the weld roots. Post-weld heat treatment or controlled cooling may be necessary to reduce residual stresses to acceptable levels.

Comparison with Conventional CFST Members

The sandwich CFST configuration offers several advantages over conventional single-tube CFST members:

The enhanced confinement and load path redundancy of the sandwich configuration come at the cost of increased fabrication complexity and welding quality requirements. The investment in fabrication quality is justified by the improved structural performance and potential for more efficient designs.

Key Insights and Reflections

This research demonstrates the analytical potential of the double shear unified strength theory for predicting the behavior of complex composite members. However, from a practical fabrication standpoint, I would emphasize several critical considerations:

1. The theoretical bearing capacity is only achievable if the concrete-steel bond is complete and uniform. Any voids or weak zones in the sandwich concrete layer can create stress concentrations that initiate cracks and lead to premature failure. The concrete placement process must be carefully monitored and verified by non-destructive testing.
2. The concentricity of the inner and outer tubes must be maintained throughout the fabrication process. Any eccentricity creates non-uniform concrete thickness, which leads to non-uniform confinement and stress concentrations. Precision assembly fixtures and in-process dimensional control are essential.
3. The welding quality of the inner tube end welds is critical but difficult to verify. These welds are concealed by the outer tube and are not accessible for conventional NDT. Advanced NDT methods or in-process weld monitoring systems must be used to ensure weld quality.
4. The long-term behavior of the sandwich concrete layer under sustained loading requires further investigation. The concrete may experience creep and shrinkage that affects the confinement pressure and, consequently, the bearing capacity. Long-term monitoring of prototype members could provide valuable data for design code development.
5. The cost-effectiveness of the sandwich configuration must be evaluated for each specific application. While the sandwich configuration offers enhanced performance, the increased fabrication cost and welding complexity may not be justified for all applications. A life-cycle cost analysis that considers both initial cost and long-term maintenance cost is recommended.

Reference Value and Outlook

The analytical framework and simplified formula presented in this paper provide a useful tool for the preliminary design of sandwich CFST columns. The agreement between calculated and experimental bearing capacities validates the approach and provides confidence in its application to practical designs. Future research should focus on cyclic loading behavior, seismic performance, and the effects of fabrication imperfections on the structural capacity of sandwich CFST members. The development of fabrication standards and welding procedure qualification requirements specific to sandwich CFST configurations would facilitate the widespread adoption of this efficient structural form in practice.