Testing and Design of Hollow Sandwich Concrete-Filled Steel Tube Members

Literature Overview

This paper by Zhou Xingyang, Xing Yuelong, Wang Jun, Chen Ju, and Guo Yong, published in Steel Structures (2017, Vol. 32, No. 3, pp. 74–77), presents experimental and analytical research on hollow sandwich concrete-filled steel tube (HSCFST) members. The specimens feature a unique twelve-sided (dodecagonal) cross-section configuration with concrete infill between inner and outer steel tubes. The research was conducted by State Grid Zhejiang Electric Power Company, Zhejiang Provincial Electric Power Design Institute, and Zhejiang University, reflecting a strong industry-academia collaboration.

Structural Configuration and Test Program

The HSCFST members consist of an outer twelve-sided steel tube, an inner twelve-sided steel tube, and concrete filling the annular space between the two tubes. This sandwich configuration provides several theoretical advantages over conventional single-tube CFST members:

The test program included specimens subjected to both axial compression and bending loads, enabling comprehensive evaluation of the member behavior under different loading conditions.

Experimental Results and Failure Modes

The experimental investigation revealed several important findings regarding the structural behavior of HSCFST members:

• Failure mode: Both inner and outer steel tubes exhibited local buckling under compressive loading. The buckling initiated at the mid-length of the column and propagated outward, with the outer tube typically buckling first due to its larger radius and lower local buckling resistance.
• Load-strain response: The members demonstrated a bilinear load-strain relationship with an initial elastic region followed by a plastic plateau, indicating good ductility and energy absorption capacity.
• Confinement effectiveness: The dual-tube configuration provided enhanced confinement to the sandwich concrete compared to single-tube CFST members, resulting in improved compressive strength and ductility of the concrete core.
• Ultimate bearing capacity: The experimental ultimate loads were compared with predictions from relevant code formulas and existing analytical models, providing validation of the design methodology.

Design Methodology

The paper addresses the design of HSCFST members through comparison with existing code provisions and analytical formulas. Key aspects of the design approach include:

• Axial compression capacity: Calculated considering the composite action of both steel tubes and the confined concrete, with appropriate interaction factors accounting for the sandwich configuration.
• Bending capacity: Evaluated using a plastic hinge approach that accounts for the non-uniform stress distribution across the composite cross-section.
• Local buckling verification: Both inner and outer tubes must satisfy local buckling limits, with the outer tube being the critical component due to its larger diameter-to-thickness ratio.

Engineering Application and Economic Benefits

The paper reports that HSCFST members have been successfully applied in actual engineering projects, achieving favorable economic benefits. The economic advantages stem from:

• Material efficiency: The sandwich configuration allows for optimized material distribution, potentially reducing overall steel consumption compared to equivalent single-tube CFST members.
• Structural performance: The enhanced confinement and composite action provide higher bearing capacity per unit weight, enabling more compact structural designs.
• Construction practicality: The dodecagonal cross-section offers a good balance between manufacturing feasibility (compared to truly circular sections) and structural efficiency (compared to multi-faceted polygonal sections).

Key Reflections and Insights

This research represents an interesting exploration of composite structural systems that go beyond conventional CFST design. The sandwich configuration is conceptually similar to the double-skin wall systems used in tunnel engineering, where an inner and outer concrete shell is connected by shear keys or steel tubes. Transferring this concept to steel-concrete composite columns is an innovative approach that deserves further investigation.

From a manufacturing perspective, the dodecagonal cross-section presents both advantages and challenges. The twelve-sided geometry can be achieved through plate rolling and welding, which is more straightforward than producing true circular tubes of large diameter. However, the welding of both inner and outer tubes, plus the potential need for shear connectors between the tubes and concrete, adds manufacturing complexity and cost.

The observation that both tubes experience local buckling is significant from a design standpoint. It implies that the inner tube, while providing additional confinement, does not eliminate the need for careful local buckling verification of both tube walls. Designers must ensure that the inner tube thickness is adequate to resist the combined effects of concrete pressure and potential buckling under compressive loads.

The successful engineering application reported in this paper is encouraging, but I would note that long-term durability considerations, particularly regarding corrosion protection of the inner tube in a concrete-filled environment, require careful attention. The aggressive alkaline environment of concrete can potentially affect the inner steel tube surface, and any corrosion-induced degradation of the inner tube would compromise the confinement effectiveness of the sandwich system.

This research opens up interesting possibilities for future structural engineering applications, particularly in applications where high compressive capacity is required in limited space, such as building columns, bridge piers, and offshore platform structures. Further research should focus on cyclic loading behavior for seismic applications, fire resistance performance, and long-term durability under various environmental conditions.