Mechanical Properties of Hexagonal Steel Tube Reinforced Concrete Columns under Bidirectional Eccentric Compression

Literature Overview

The paper by Wu Hongjun, Wang Zhibin, and Lin Xin (2019), published in Industrial Construction (工业建筑), Vol. 49, No. 4, pp. 164-168, investigates the mechanical behavior of hexagonal steel tube reinforced concrete (STRC) columns under bidirectional eccentric compression. The research was supported by the Fujian Provincial Natural Science Foundation (2017J01696) and the Fuzhou Science and Technology Program (2017-G-101). The study employed finite element analysis using ABAQUS software to simulate the load-deformation behavior and conduct parametric analysis.

Core Technical Findings

The finite element analysis revealed several important mechanical characteristics of hexagonal STRC columns under bidirectional eccentric compression:

Confinement Mechanism Analysis

The hexagonal cross-section presents a unique confinement geometry compared to conventional circular or rectangular sections. The confinement effectiveness varies around the perimeter of the hexagon:

1. Corner regions: The internal angles of a regular hexagon are 120 degrees. At the corners, the steel tube provides concentrated lateral restraint to the concrete core. The geometric configuration at the corners creates a more effective confinement zone, similar to the behavior observed in rectangular STRC columns where corner confinement is superior.
2. Flat side regions: Along the flat sides of the hexagon, the confinement pressure is more uniform but generally lower than at the corners. The concrete in these regions experiences less lateral restraint, leading to earlier crushing and reduced confinement effectiveness.
3. Transition regions: Between the corners and flat sides, the confinement pressure varies continuously, creating a gradient of confinement effectiveness around the cross-section.

This non-uniform confinement distribution has implications for the stress-strain behavior of the confined concrete. The concrete at the corners will experience higher confining pressures and therefore higher ultimate compressive strains, while the concrete along the flat sides will reach its crushing strain earlier.

Moment Interaction Curve Analysis

The shape of the moment interaction curve (Mx-My diagram) is a critical design parameter for columns subjected to combined bending in two directions. The study found that:

• Low axial compression ratio: The interaction curve approximates a quarter circle, indicating that the column's bending capacity is relatively independent of the bending direction. This suggests that the confinement effect is relatively uniform and the column behaves similarly in both bending directions.
• High axial compression ratio: The interaction curve transitions to a quarter ellipse, indicating a more pronounced anisotropy in bending capacity. The elongation of the ellipse reflects the increased influence of the axial load on the bending capacity, with the major axis of the ellipse aligning with the direction of greater confinement effectiveness.

Simplified Calculation Method

The authors proposed a simplified calculation formula for the load capacity of hexagonal STRC columns under bidirectional eccentric compression. The formula was validated against finite element results and found to be conservative, making it suitable for engineering design purposes. The conservative nature of the formula provides a safety margin, which is particularly important for columns in seismic regions where the exact loading conditions are uncertain.

The simplified formula likely incorporates the following parameters:

• Geometric properties of the hexagonal cross-section (side length, wall thickness)
• Material properties of the steel tube and concrete
• Axial compression ratio
• Eccentricity ratios in both bending directions

Engineering Practice Considerations

Hexagonal STRC columns offer several advantages for practical applications:

1. Architectural flexibility: The hexagonal shape provides aesthetic appeal and can be integrated into architectural designs that require non-conventional column shapes.
2. Material efficiency: Compared to circular sections of equivalent area, hexagonal sections may offer improved structural efficiency in certain loading conditions due to the corner confinement effect.
3. Fabrication feasibility: Hexagonal steel tubes can be fabricated by rolling and welding flat plates, which is a well-established manufacturing process. The welding of the six longitudinal seams requires careful control of welding parameters to ensure consistent quality.
4. Connection detailing: The flat sides of the hexagonal section provide convenient surfaces for connection plates, which simplifies the connection design compared to circular sections.

Welding and Fabrication Challenges

From a fabrication perspective, hexagonal steel tubes present several challenges:

• Seam welding quality: Six longitudinal weld seams must be produced with consistent quality. Each seam is subject to the same welding metallurgy concerns as conventional pipe welds, including HAZ sensitization, residual stress, and distortion.
• Corner integrity: The corners of the hexagonal section experience complex stress states during both fabrication and service. The welds at or near the corners must be carefully inspected to ensure no defects are present.
• Dimensional accuracy: The regularity of the hexagonal shape is critical for the confinement mechanism to function as expected. Deviations from the ideal hexagonal geometry can significantly affect the structural performance.

Study Insights and Implications

This research contributes to the understanding of non-circular STRC columns and provides a basis for the design of hexagonal STRC columns in practical engineering applications. The finding that corner regions provide superior confinement is consistent with observations in other non-circular sections and reinforces the importance of geometric configuration in composite column design.

The simplified calculation formula, while conservative, provides a practical tool for preliminary design and can be used in conjunction with more detailed finite element analysis for final design verification. The conservative bias is acceptable for engineering design, particularly when combined with appropriate safety factors.

The study highlights the potential of hexagonal STRC columns as a viable alternative to conventional circular and rectangular STRC columns, particularly in applications where architectural requirements or structural efficiency considerations favor non-circular sections. Future research should focus on experimental validation of the finite element predictions and the development of design provisions for hexagonal STRC columns in relevant building codes.