Mechanical Properties of Hexagonal Steel Pipe Concrete Bidirectional Eccentric Compression Columns

Literature Overview

The paper by Wu Hongjun, Wang Zhibin, and Lin Xin, published in Industrial Construction (2019, Vol. 49, No. 4, pp. 164-168), investigates the mechanical behavior of hexagonal steel pipe concrete columns under bidirectional eccentric compression loading. The research was funded by the Fujian Provincial Natural Science Foundation (2017J01696) and the Fuzhou Science and Technology Program (2017-G-101).

Core Technical Content

The authors established constitutive models for steel and concrete, used ABAQUS finite element software to simulate the load-deformation behavior of typical hexagonal steel pipe concrete bidirectional eccentric compression members, and conducted mechanism analysis and parametric analysis. They also proposed a simplified calculation formula for the bearing capacity of such members.

Key Findings

Interpretation of Technical Points

Hexagonal steel pipe concrete columns represent a geometric innovation that combines the structural efficiency of steel pipe concrete with the architectural and functional advantages of a polygonal cross-section. The hexagonal shape provides a larger cross-sectional area than a circular section of the same circumscribed diameter, while maintaining a relatively compact profile that is advantageous in certain architectural applications.

Confinement Mechanism Analysis

The finding that corner regions provide higher confinement than flat segments is mechanistically sound. At the corners of a hexagonal section, the steel tube walls meet at an angle, creating a geometric constraint that enhances the lateral confinement pressure on the enclosed concrete. In contrast, flat segments provide uniform but lower confinement pressure. This non-uniform confinement distribution has implications for the stress-strain behavior of the concrete core and the overall load-deformation response of the column.

The parametric analysis reveals an important transition in the moment interaction curve shape: at low axial compression ratios, the interaction curve is approximately circular, indicating that the column's bending capacity is relatively isotropic. At higher axial compression ratios, the curve becomes elliptical, reflecting the anisotropic bending capacity inherent in the hexagonal geometry. This is because the hexagonal section has different moments of inertia about different axes, and this difference becomes more pronounced at higher axial loads where the plastic zone distribution is more sensitive to section geometry.

Constitutive Model Selection

The constitutive models for steel and concrete used in the finite element analysis are critical to the accuracy of the results. For the steel tube, a bilinear or multi-linear elastic-plastic model with strain hardening is typically appropriate. For the confined concrete, a model that accounts for the lateral confinement pressure (such as the Mander model or a modified version thereof) is necessary to capture the enhanced compressive strength and ductility of the concrete core.

Engineering Practice Considerations

For structural engineers considering the use of hexagonal steel pipe concrete columns, the following points are relevant:

1. Fabrication complexity: Hexagonal steel pipes require specialized forming processes. The corner regions may experience higher strain during forming, which could affect the local mechanical properties and residual stress distribution.
2. Concrete placement: The hexagonal cross-section presents challenges for concrete pouring and vibration. The corner regions are particularly difficult to fill completely, and voids in these regions would compromise the confinement effectiveness.
3. Connection design: The hexagonal geometry affects the design of connections at the column ends. The varying section depth in different directions requires careful detailing of beam-column connections and base plates.

Key Questions and Reflections

A key question is whether the simplified calculation formula proposed in the study is sufficiently accurate for practical design use. The authors note that the simplified results are conservative relative to the finite element analysis, which is generally acceptable from a safety perspective. However, excessive conservatism may lead to uneconomical designs, particularly for large-scale structures where material efficiency is important.

Another reflection concerns the comparison between hexagonal and other polygonal sections (such as octagonal or dodecagonal). As the number of sides increases, the section approaches a circular shape, and the confinement becomes more uniform. The trade-off between the geometric efficiency of a hexagonal section and the more uniform behavior of a circular section should be evaluated for each specific application.

Study Insights and Implications

This research contributes to the understanding of non-circular steel pipe concrete columns and provides practical tools for their design. The identification of the transition from circular to elliptical moment interaction curves based on axial compression ratio is a valuable insight for engineers performing biaxial bending analysis. The proposed simplified calculation formula, while conservative, offers a practical alternative to full finite element analysis for preliminary design and code compliance checks. The study also highlights the importance of constitutive model selection in capturing the complex interaction between steel confinement and concrete behavior in non-circular sections.