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

Literature Overview

The paper by Wu Hongjun, Wang Zhibin, and Hao Huailin (2019), published in the Journal of Fuzhou University (Natural Science Edition), investigates the mechanical behavior of round-ended steel tube concrete (STC) columns under bidirectional eccentric compression. Using the finite element software ABAQUS, the authors simulated the load-deformation curves of typical round-ended STC columns and conducted a detailed analysis of the working mechanism. The study also proposes a simplified calculation formula for the bearing capacity of such columns.

Core Technical Findings

The study demonstrates that round-ended STC columns exhibit high ultimate bearing capacity and ductility under bidirectional eccentric loading. The steel tube provides effective confinement to the core concrete, with the arc segments of the round-ended section offering superior confinement effects compared to the flat segments. The interaction curve of $M_x/M_{ux}$ versus $M_y/M_{uy}$ takes the shape of a quarter ellipse.

Confinement Mechanism Analysis

The confinement effect is a fundamental concept in composite column design. In a round-ended section, the geometry creates a unique stress distribution pattern:

The arc segments, due to their curved geometry, distribute the confining pressure more uniformly across the concrete core surface. This is analogous to the well-established confinement behavior in circular steel tube columns, where the hoop stress in the steel tube provides uniform radial confinement. In contrast, flat segments experience non-uniform contact pressure, with higher stresses near the corners and lower stresses at the mid-span of the flat segment.

Finite Element Modeling Considerations

The authors employed appropriate constitutive models for both steel and concrete, which is critical for accurate simulation of the nonlinear behavior. Key modeling aspects include:

• Steel constitutive model: Elastic-perfectly plastic or bilinear kinematic hardening, accounting for the von Mises yield criterion and isotropic hardening behavior.
• Concrete constitutive model: A damage-based plasticity model or a confined concrete model (such as Mander model or Kent-Park model) that captures the strength and ductility enhancement under triaxial compression.
• Interface modeling: The bond-slip behavior between steel and concrete must be accurately represented, typically using cohesive elements or penalty contact formulations.

The quarter-elliptical interaction curve is a significant finding for practical design purposes. This means that the biaxial bending capacity can be interpolated using a simple elliptical formula, which simplifies the design process considerably compared to complex multi-parameter interaction curves.

Simplified Calculation Formula and Comparison

The proposed simplified calculation formula yields results that are conservative relative to the finite element simulation results. This conservatism is acceptable from a safety standpoint but may lead to over-design in practical applications. The ratio of the simplified result to the FE result can be used as a calibration factor in preliminary design stages.

Engineering Practice Implications

For steel pipe and fitting manufacturers, the round-ended section geometry presents specific fabrication challenges:

• The transition between arc and flat segments requires precision forming, typically achieved through roll forming or press bending processes.
• Welding at the transition zones demands careful heat input control to prevent distortion and residual stress accumulation.
• The quality of the weld seam in the round-ended section directly affects the confinement effectiveness, as any geometric discontinuity can create stress concentrations.

The finding that arc segments provide superior confinement suggests that optimizing the arc-to-flat ratio in the cross-section could enhance structural performance. This has direct implications for the design of custom-shaped steel tubes used in composite columns, where the section geometry can be tailored to maximize confinement efficiency.

Study Insights and Outlook

This research provides valuable insights into the behavior of non-circular steel tube concrete columns under complex loading conditions. The quarter-elliptical interaction curve simplifies design calculations significantly, while the confinement mechanism analysis offers guidance for section optimization. Future research should investigate the effect of different arc-to-flat ratios, the influence of steel tube wall thickness variations, and the long-term behavior under sustained loading with creep and shrinkage effects. For fabrication engineers, the study highlights that geometric precision in round-ended tube manufacturing is not merely a dimensional requirement but a structural performance factor that directly influences the confinement effectiveness and ultimate capacity of composite columns.