Eccentric Compression Behavior of Composite Thin-Walled Square Steel Tube Concrete Columns with Longitudinal Stiffeners

Literature Overview

The paper by Wang Zhibin, Wei Feibao, Chi Siyuan, and Yu Xin, published in the Journal of Hunan University (Natural Science Edition) in 2018, presents an experimental and analytical study of the eccentric compression behavior of composite thin-walled square steel tube concrete (CSTSC) columns with longitudinal stiffeners. This research was supported by the National Natural Science Foundation of China and other funding agencies. The study combines experimental testing, finite element analysis, and parametric studies to develop simplified calculation formulas for the bearing capacity of these composite columns.

Core Technical Methodology

The experimental program consisted of 9 CSTSC columns with longitudinal stiffeners subjected to eccentric compression loading. The test specimens were designed to investigate the effects of eccentricity ratio, slenderness ratio, and concrete strength on the structural behavior and failure modes. The experimental results were used to validate a finite element model, which was then employed for extensive parametric studies to explore the structural behavior under various conditions.

The finite element model was developed in a commercial structural analysis software package and was validated against the experimental test results. Once validated, the model was used to conduct parametric studies varying parameters such as steel yield strength, inner steel tube concrete steel ratio, width-to-thickness ratio, concrete strength, and slenderness ratio. The parametric studies provided insights into the structural mechanisms and the relative importance of different design parameters.

Key Experimental Findings

The experimental results reveal several important trends in the behavior of CSTSC columns under eccentric compression:

The study also identifies several important structural mechanisms. The outer steel tube provides confinement primarily concentrated at the corners of the square section, while the concrete core carries the majority of the axial load. The inner steel tube concrete fills the core and can improve the ductility of the composite member.

Finite Element Analysis and Parametric Studies

The validated finite element model was used to conduct extensive parametric studies, which revealed several important design trends:

1. As steel yield strength increases, the equilibrium point of the normalized axial force-moment interaction curve moves to lower coordinates, indicating a shift in the balance between axial and flexural capacity.
2. As the inner steel tube concrete steel ratio increases, the equilibrium point coordinates decrease, suggesting that the inner steel tube concrete contributes more to the flexural capacity than the axial capacity.
3. As the width-to-thickness ratio increases, the equilibrium point coordinates decrease, indicating reduced local buckling resistance.
4. As concrete strength increases, both the horizontal and vertical coordinates of the equilibrium point increase, indicating improved overall capacity.
5. As slenderness ratio increases, the axial force-moment interaction curve approaches a straight line, indicating a transition from material failure to buckling-dominated behavior.

Process Analysis and Welding Considerations

From a steel pipe and welding perspective, the CSTSC column configuration presents several important fabrication and welding challenges. The outer square steel tube is typically fabricated from flat steel plates welded into a square cross-section using fillet welds or groove welds. The longitudinal stiffeners are welded to the internal surface of the outer tube, and the inner steel tube concrete is placed inside the outer tube.

The welding quality at several critical locations is paramount for the structural integrity of the CSTSC column:

• Longitudinal stiffener welds: These welds connect the stiffeners to the outer tube and must resist the compressive forces and potential buckling stresses. Any weld defects such as lack of fusion, porosity, or cracks can initiate local buckling at the stiffener-to-tube junction.
• Outer tube corner welds: The corner welds of the square tube must be full-penetration groove welds with thorough NDT inspection to ensure complete fusion and the absence of defects.
• Inner tube connections: If the inner steel tube is connected to the outer tube at the ends, these connections must be designed to transfer the axial forces and moments between the inner and outer tubes.

For the NDT of these welds, ultrasonic testing (UT) is the preferred method for detecting internal defects such as lack of fusion, porosity, and inclusions. Magnetic particle testing (MT) is effective for detecting surface and near-surface defects such as cracks and laps. Radiographic testing (RT) provides detailed images of the weld cross-section but is limited by accessibility and cost considerations.

Simplified Calculation Formulas

Based on the parametric studies, the authors propose simplified calculation formulas for the bearing capacity of CSTSC columns under eccentric compression. These formulas account for the effects of eccentricity ratio, slenderness ratio, steel yield strength, concrete strength, and the geometric properties of the composite section. The formulas provide a practical tool for engineers to design CSTSC columns without the need for detailed finite element analysis.

The simplified formulas should be used with caution, as they are based on parametric studies of a limited number of configurations. Engineers should verify the applicability of the formulas for specific design conditions and consider using finite element analysis for critical or unconventional configurations.

Key Questions and Reflections

A significant question raised by this research is the long-term behavior of CSTSC columns under sustained loading and cyclic loading conditions. The experimental tests were conducted under monotonic eccentric compression loading, which may not fully capture the behavior under sustained loads with creep and shrinkage effects or under cyclic loading with fatigue and low-cycle fatigue effects.

Another important consideration is the interaction between local buckling of the steel tube walls and the overall column buckling. The parametric studies indicate that the width-to-thickness ratio significantly affects the structural behavior, and engineers must ensure that the steel tube walls are adequately stiffened to prevent premature local buckling that could compromise the overall column capacity.

Study Insights and Implications

This research provides valuable insights into the structural behavior of CSTSC columns with longitudinal stiffeners under eccentric compression, offering practical design tools for engineers. The experimental and analytical results demonstrate that the composite action between the outer steel tube, the longitudinal stiffeners, and the inner steel tube concrete can significantly improve the bearing capacity and ductility of the columns.

For steel pipe manufacturers and welding engineers, the study highlights the importance of maintaining high-quality welding at all critical locations in CSTSC columns. The structural integrity of the composite column depends on the quality of the welds connecting the stiffeners to the outer tube and the connections between the inner and outer tubes. Rigorous NDT inspection and quality control procedures are essential to ensure the reliability of these critical welds.

In conclusion, this study provides a comprehensive understanding of the eccentric compression behavior of CSTSC columns with longitudinal stiffeners, offering validated finite element models and simplified calculation formulas that enable engineers to design these efficient composite columns with confidence while ensuring the quality of the critical welds that define their structural performance.