Axial Compression Behavior of Circular CFST Columns with Embedded High-Strength Steel Tubes

Research Background and Significance

The paper by Wu Bo and Ye Wenjie (2020, Journal of South China University of Technology, Vol. 48, No. 4) investigates a novel hybrid CFST column configuration that incorporates an internal high-strength steel tube within the outer steel tube. Funded by the National Key R&D Program (2017YFC0703303) and the National Natural Science Foundation (51778240), this work explores how the strategic placement of high-strength steel inside a conventional CFST column can enhance axial load capacity without increasing total steel consumption. This concept is particularly relevant in the context of resource efficiency and structural optimization.

Experimental Program

Twelve circular CFST column specimens were tested under monotonic axial compression. The test variables and their ranges are presented below:

Key Experimental Findings

1. Total steel efficiency: By appropriately thinning the outer steel tube and incorporating a high-strength internal tube, the axial load capacity of the circular CFST column is significantly improved while maintaining approximately the same total steel tonnage. This is a direct demonstration of material optimization through strategic steel grade selection.
2. Initial stiffness enhancement: The presence of the internal high-strength tube increases the initial stiffness of the column, which is beneficial for serviceability design and seismic performance.
3. Net gap effect: Increasing the net gap between the inner and outer tubes further enhances the axial load capacity. This is attributed to improved concrete confinement and better stress distribution in the annular concrete region.
4. Internal tube steel ratio: Changes in the steel ratio of the internal tube have minimal effect on the overall axial compression behavior, suggesting that the benefit is primarily from the confinement geometry rather than the additional steel mass.
5. Recycled concrete: The use of recycled concrete aggregate blocks has negligible impact on the axial compression behavior of this column type, supporting the adoption of sustainable construction materials.

Technical Analysis

Confinement Mechanism

The hybrid inner-outer tube configuration creates a unique confinement geometry. The outer tube provides primary confinement to the core concrete, while the inner tube provides secondary confinement and acts as a structural reinforcement element. The annular concrete between the two tubes experiences triaxial compression, which mobilizes the concrete's full compressive potential. The net gap size directly influences the volume of this highly confined annular concrete, explaining the observed capacity improvement with larger gaps.

Welding and Fabrication Considerations

The fabrication of hybrid CFST columns with embedded inner tubes introduces several welding challenges:

• Tie welds: The inner and outer tubes must be connected at regular intervals through tie welds or through-plate connections to ensure composite action. These welds must be designed to transfer the differential deformation between the two tubes.
• Tube-to-tube clearance: The net gap must accommodate shrinkage of the concrete and differential thermal expansion. A minimum gap of 50-80 mm is typically specified.
• Concrete placement: The annular gap between tubes requires careful concrete placement to ensure full compaction. Vibrators of appropriate diameter must be used, and the concrete mix must have sufficient workability.

Bearing Capacity Calculation

The authors propose an improved axial bearing capacity calculation method that accounts for the confinement effect of both tubes. The method shows good predictive accuracy across the test database. From a practical standpoint, engineers should verify that the proposed formula is compatible with the design code provisions in use (e.g., GB 50936, CECS 38, or Eurocode 4).

Study Insights

This research is highly relevant to current industry trends toward structural efficiency and sustainability. The finding that recycled concrete can be used without significant performance degradation is particularly encouraging for green building initiatives. From a metallurgical and welding perspective, the use of high-strength internal tubes (such as Q690 or even S960 grades) requires careful attention to weldability. These grades have higher carbon equivalent values (CEV > 0.55), necessitating preheating, controlled heat input, and post-weld heat treatment for tie welds and end connections. The welding procedure specification (WPS) must be qualified in accordance with ISO 15614-1 or GB/T 985.1, with particular attention to hydrogen-induced cracking prevention.

Conclusion

The hybrid CFST column with embedded high-strength inner tube represents a promising structural concept that achieves higher axial capacity through intelligent steel distribution rather than increased steel tonnage. The findings on net gap optimization, steel ratio insensitivity, and recycled concrete compatibility provide a solid foundation for practical design. Engineers should pursue further research on seismic performance, fire resistance, and fatigue behavior of this column type before widespread adoption in critical infrastructure.