Experimental Study on Eccentrically Loaded Composite Steel Tube Concrete Columns

Literature Overview

Ma Shufang, Guo Hongxiang, Zhao Junhai, and Wei Xueying (2007), published in Industrial Construction, present experimental results on eccentrically loaded composite steel tube concrete (CSTC) columns with innovative cross-section configurations. Funded by the Ministry of Education Doctoral Point Fund and the Shaanxi Provincial Natural Science Foundation, this research from Chang'an University investigates two distinct composite cross-section types: internally reinforced square steel tube concrete columns with embedded circular steel tubes, and square hollow sandwich CSTC columns.

The study examined 9 specimens with varying dimensions and wall thicknesses, providing valuable data on the load-bearing capacity and load-deformation behavior of these composite sections under eccentric compression.

Specimen Configuration and Test Setup

The experimental program included two categories of specimens:

The eccentric compression tests were conducted to evaluate how the presence of internal circular steel tubes affects the structural performance of the composite columns. The eccentricity ratio was varied to simulate realistic loading conditions encountered in structural applications.

Core Experimental Findings

Internal Circular Reinforcement Type

The most significant finding from the internal circular reinforcement specimens is that the embedded circular steel tube provides a strengthening effect on the core concrete of the square steel tube concrete column under eccentric loading. This strengthening effect becomes more pronounced as the diameter of the circular tube increases.

The mechanism behind this strengthening can be understood through the confinement interaction between the circular tube and the surrounding concrete. Under eccentric compression, the concrete on the compression side experiences higher stresses and tends to expand laterally. The circular steel tube resists this expansion, providing additional confinement that increases the compressive strength and ductility of the concrete in the critical region.

Square Hollow Sandwich Type

In contrast, for the square hollow sandwich CSTC columns, increasing the circular tube diameter leads to a greater reduction in load-bearing capacity. This counterintuitive result can be explained by the different structural mechanism in this cross-section type. In the hollow sandwich configuration, the circular tube creates a void within the concrete section, reducing the effective concrete area. As the circular tube diameter increases, the loss of concrete area becomes more significant, and the remaining concrete is less effectively confined by the outer square tube due to the increased distance between the concrete and the confining steel.

Technical Analysis of Eccentric Loading Behavior

Eccentric compression subjects the column to combined axial and bending effects. The load-deformation curves obtained from the tests reveal important characteristics of the composite section behavior:

1. Initial elastic stage: The column behaves elastically with linear load-deformation response. The stiffness is higher for the internal reinforcement type due to the increased moment of inertia from the embedded circular tube.
2. Yielding stage: The steel tubes begin to yield, with the outer square tube yielding first due to the higher stresses at the outer fibers. The internal circular tube yields later due to the lower stress level at its position.
3. Post-yield stage: The concrete continues to carry increasing load through confinement effects. The internal circular tube provides additional confinement to the concrete, delaying crushing and extending the post-yield capacity.
4. Failure stage: Failure initiates at the most highly stressed concrete region and propagates through the section. The presence of the internal circular tube modifies the failure pattern by redistributing stresses and providing additional load paths.

Comparison of the Two Cross-Section Types

The contrasting behavior of the two cross-section types highlights the importance of structural configuration in determining composite member performance:

The internal reinforcement type demonstrates superior structural performance because the circular tube actively participates in the load-bearing mechanism through direct confinement of the concrete. The hollow sandwich type, by contrast, loses concrete volume without gaining proportional structural benefit, resulting in net capacity reduction.

Engineering Practice Considerations

For steel pipe manufacturers and structural engineers, this research provides important guidance on the design of composite CSTC columns:

• Material selection: The circular reinforcement tube should be selected to complement the outer square tube in terms of material grade and wall thickness. Matching the material properties ensures efficient load sharing between the tubes.
• Fabrication quality: The placement of the internal circular tube within the square tube requires precise alignment and secure positioning before concrete placement. Misalignment can significantly reduce the confinement effectiveness.
• Welding considerations: If the internal and external tubes are connected by welding, the weld quality is critical. Insufficient weld penetration or defects can create stress concentrations that initiate premature failure.
• Concrete placement: The concrete must be thoroughly compacted around the internal circular tube to ensure full bond and confinement. Vibration-assisted placement is recommended to achieve adequate compaction in the confined space between the tubes.

Key Questions and Reflections

Several aspects of the research warrant further investigation:

1. How does the spacing between the internal circular tube and the outer square tube affect the confinement effectiveness?
2. What is the behavior of these composite sections under cyclic loading, relevant to seismic applications?
3. Can the hollow sandwich type be improved by adding internal reinforcement or by using high-performance concrete in the reduced section?
4. How do manufacturing tolerances and construction quality variations affect the predicted structural performance?

The research demonstrates that the structural configuration of composite CSTC columns has a profound influence on their load-bearing capacity and deformation characteristics. The distinction between active reinforcement (internal circular tube in contact with concrete) and passive void (hollow sandwich configuration) is a critical design consideration that should be clearly communicated to practicing engineers.

Study Insights and Practical Recommendations

This experimental study provides valuable data on the eccentric compression behavior of innovative CSTC cross-section configurations. The key insight is that the presence of an internal circular steel tube does not universally improve structural performance; rather, the benefit depends on whether the tube actively participates in the load-bearing mechanism through concrete confinement.

For engineering practice, the internal reinforcement type is recommended for applications where increased load capacity and ductility are required. The hollow sandwich type should be used with caution and only when specific design considerations justify the reduced concrete volume. In both cases, careful attention to fabrication quality, concrete placement, and connection details is essential to achieve the predicted structural performance.

The load-deformation curves obtained from the tests provide a basis for developing design formulas for these composite sections. Future research should extend these findings to include parametric studies, numerical modeling, and the development of practical design recommendations for use in engineering codes.