Mechanical Properties of Round-Ended Concrete-Filled Steel Tube Columns Under Bidirectional Eccentric Compression

Literature Overview

This paper by Wu Hongjun, Wang Zhibin, and Hao Huailin from Fuzhou University, published in Journal of Fuzhou University (Natural Science Edition) (2019, Vol. 47, No. 4, pp. 515-520), investigates the structural behavior of round-ended CFST columns subjected to bidirectional eccentric compression. The research employs finite element analysis (FEA) using ABAQUS to simulate the load-deformation response and proposes simplified calculation formulas for practical design.

The round-ended cross-section is a relatively novel geometry that combines the advantages of circular CFST columns (uniform confinement, efficient material utilization) with the practical benefits of flat segments (easier connection detailing, improved constructability).

Core Technical Methodology

Finite Element Modeling Approach

The constitutive models for steel and concrete are carefully selected to capture the confinement effect accurately. The confined concrete model accounts for the lateral restraint provided by the steel tube, which increases both the compressive strength and ductility of the core concrete.

Cross-Sectional Geometry Analysis

The round-ended cross-section consists of:

• Two flat segments (straight portions)
• Two circular arc segments (rounded ends)

This geometry creates a non-uniform confinement effect:

Key Findings and Technical Interpretation

Load-Bearing Capacity and Ductility

The round-ended CFST columns demonstrate:

• High ultimate load-bearing capacity: The combination of steel tube strength and confined concrete strength provides substantial axial resistance.
• Excellent ductility: The steel tube prevents concrete spalling and maintains load-carrying capacity beyond the concrete crushing point.
• Effective confinement: The steel tube provides significant lateral restraint to the core concrete, particularly in the circular arc regions.

Interaction Curve Characteristics

A notable finding is that the interaction curve (Mx/Mux vs. My/Muy) for the round-ended CFST column under bidirectional eccentric compression takes the form of a quarter ellipse. This is a significant result for design purposes because:

1. The elliptical shape provides a clear mathematical relationship between biaxial bending moments
2. The quarter-ellipse form can be readily incorporated into design codes
3. The shape is independent of the specific cross-sectional dimensions within certain parametric ranges

Simplified Calculation Formula

The authors propose a simplified calculation formula for the load-bearing capacity of round-ended CFST columns under bidirectional eccentric compression. The formula is validated against FEA results and shows conservative predictions, which is desirable from a safety perspective.

Connection to Steel Pipe Engineering Practice

Tube Fabrication Considerations

The round-ended cross-section poses specific challenges for steel tube manufacturing:

The welding quality of the tube walls is critical because:

• Weld seams introduce geometric discontinuities that may affect confinement uniformity
• HAZ properties influence the overall ductility of the column
• Residual stresses from welding can trigger premature local buckling

Material Requirements

For round-ended CFST columns, the following material properties are particularly important:

• Steel tube: Adequate elongation (≥25% for Q345) to accommodate plastic deformation
• Core concrete: Controlled slump to ensure proper placement within the tube
• Bond quality: Sufficient steel-concrete interface adhesion for load transfer

Welding Process Recommendations

Based on the structural behavior revealed by this study, the following welding practices are recommended:

1. Use GTAW for root passes to ensure full penetration
2. Apply SAW or GMAW for fill and cap passes to improve deposition efficiency
3. Maintain interpass temperature below 200°C to avoid excessive grain growth
4. Implement post-weld stress relief for critical applications
5. Conduct UT inspection of weld seams per ASME B31.3 or equivalent standards

Critical Reflection and Study Insights

The quarter-ellipse interaction curve is an elegant result that simplifies the design of round-ended CFST columns under biaxial bending. However, I note several areas where further investigation would be beneficial:

• The study focuses on typical specimens; parametric studies varying the ratio of flat segment length to arc radius would provide more comprehensive design guidance.
• The effect of concrete strength grade on the interaction curve shape should be quantified.
• Cyclic loading behavior (important for seismic applications) is not addressed.
• The transition from elastic to plastic behavior under bidirectional eccentric compression deserves more detailed discussion.

From a steel pipe quality control perspective, the non-uniform confinement effect highlights the importance of:

• Ensuring dimensional accuracy of the tube profile
• Controlling weld geometry to minimize stress concentrations
• Verifying tube wall thickness uniformity through UT or EMAT testing

Reference Value and Outlook

This research provides valuable design guidance for round-ended CFST columns, which represent an emerging structural solution combining constructability with structural efficiency. The proposed simplified formula, while conservative, offers a practical tool for engineers to evaluate these columns under complex loading conditions.

For steel pipe manufacturers, the growing adoption of non-circular CFST cross-sections presents both challenges and opportunities. The ability to produce high-quality round-ended tubes with consistent welding quality will be a competitive advantage in the CFST structural market. Standards bodies should consider incorporating the design methods developed in this research into future code revisions.