Testing and Design of Hollow Double-Layer Concrete-Filled Steel Tube Members

Literature Overview

This paper by Zhou Xingyang, Xing Yuelong, Wang Jun, Chen Ju, and Guo Yong, published in Steel Construction (2017, Vol. 32, No. 3, pp. 74-77), presents experimental and analytical research on hollow double-layer concrete-filled steel tube (HDCFST) members with dodecagonal cross-sections. The research was conducted collaboratively between State Grid Zhejiang Electric Power Company, Zhejiang Electric Power Design Institute, and Zhejiang University, reflecting a strong industry-academia partnership.

Structural Configuration and Design Philosophy

The HDCFST system consists of an outer dodecagonal steel tube and an inner dodecagonal steel tube with concrete filled in the annular space between them. This configuration differs from conventional concrete-filled steel tubes in several fundamental ways:

• The concrete is confined between two steel tubes rather than being fully enclosed by a single tube
• Both the inner and outer steel tubes contribute to load-bearing capacity
• The hollow center reduces self-weight while maintaining structural integrity
• The dodecagonal cross-section approximates a circular section while facilitating fabrication

The dodecagonal geometry was selected because it provides a good balance between structural efficiency (approaching circular section performance) and fabrication practicality (using flat plates with fewer curved segments than a circular tube).

Experimental Findings

The test results revealed several important mechanical behaviors:

1. Failure Mode: Both the inner and outer steel tubes exhibit local compressive buckling, which is a critical finding because it indicates that neither tube can be assumed to remain elastic throughout the loading process.
2. Load-Displacement Behavior: The members demonstrate significant ductility with a clear post-peak load capacity, indicating that the concrete annulus continues to contribute to load-bearing even after initial buckling.
3. Comparison with Codes: The experimental results were compared with existing code formulas, revealing discrepancies that highlight the need for specific design provisions for this novel structural system.

Design Methodology Development

Based on the experimental data, the authors developed design formulas for the ultimate load capacity under both axial compression and bending. The key design considerations include:

• Axial Compression Capacity: The total capacity is the sum of contributions from the outer tube, inner tube, and concrete annulus, with appropriate reduction factors for local buckling effects.
• Bending Capacity: The interaction between axial force and bending moment requires consideration of the different stress distributions in the outer and inner tubes.
• Local Buckling Resistance: Both tubes require adequate plate buckling resistance, which depends on the diameter-to-thickness ratio of each tube.

Engineering Application and Economic Benefits

The paper reports successful application of the HDCFST system in actual engineering projects, demonstrating significant economic advantages. The hollow center reduces material consumption while the dual-tube configuration provides enhanced structural performance. This makes the system particularly suitable for applications where self-weight is a critical design constraint, such as transmission tower structures and large-span bridges.

The economic benefits arise from several factors: reduced concrete volume (and therefore reduced self-weight), optimized use of steel in both tubes, and simplified construction due to the use of flat plate segments. The successful field application validates the theoretical and experimental findings and provides confidence for wider adoption.

Key Technical Challenges

Several technical challenges remain for the practical implementation of HDCFST members:

• Concrete Placement: Ensuring proper concrete compaction in the annular space between two tubes requires careful construction sequencing and appropriate vibrator access.
• Interface Bond: The bond between the concrete and both steel tubes is critical for composite action, and the annular geometry may present challenges for achieving uniform bond strength.
• Fabrication Tolerances: The dodecagonal geometry requires precise fabrication of both tubes to ensure proper alignment and uniform concrete thickness in the annular space.

Summary

This research contributes valuable experimental data and design methodology for a novel structural system that offers significant economic and performance advantages. The successful engineering application demonstrates the practical viability of HDCFST members, and the findings provide a foundation for future development of design codes and standards for this structural system.