Simplified Method for Determining Plastic Yield Surface of Square CFST Flexural-Compressive Members

Literature Overview

This paper by Liu Yangbing, Liu Jingbo, Han Qiang, and Zheng Nina, published in 2010 in the Journal of Chongqing University (Natural Science Edition), presents a simplified method for determining the plastic yield surface of square concrete-filled steel tube (CFST) flexural-compressive members. The study was supported by multiple funding sources, including the National Natural Science Foundation of China, reflecting its academic significance. The authors propose a rapid method for determining the axial force-bending moment interaction yield surface, which is essential for plastic hinge analysis of CFST structures but is often computationally intensive when using the fiber model method.

Core Technical Points

The plastic yield surface defines the boundary between elastic and plastic behavior of a structural member under combined axial and bending loads. For CFST members, the yield surface is complex due to the interaction between the steel tube and the concrete core, the confinement effect of the steel tube on the concrete, and the non-linear stress-strain behavior of both materials. The fiber model method, which divides the cross-section into discrete fibers and calculates the stress in each fiber based on the strain distribution, provides accurate results but is computationally intensive and not convenient for practical engineering applications.

The proposed simplified method involves two main steps: first, the elastic element parameters are determined through theoretical analysis; second, the plastic yield surface is rapidly determined through theoretical analysis and extensive parametric studies. The method is validated by comparing the results with those obtained from the fiber model method, showing good agreement.

The plastic yield surface for a square CFST section is characterized by the interaction between the axial force $N$ and the bending moment $M$. The yield surface is typically plotted in the $N$-$M$ plane, with the maximum axial force capacity ($N_0$) and the maximum bending moment capacity ($M_0$) defining the intercepts. The shape of the yield surface between these intercepts is determined by the material properties, section geometry, and the confinement effect.

Interpretation of Technical Significance

The simplified method addresses a practical need in the design and analysis of CFST structures. In seismic design, the plastic hinge method is commonly used to analyze the non-linear behavior of structures, and the plastic yield surface is a fundamental input for this analysis. The fiber model method, while accurate, requires significant computational resources and expertise, making it impractical for routine engineering design. The proposed simplified method provides a practical alternative that maintains acceptable accuracy while significantly reducing the computational effort.

For steel pipe engineers, the plastic yield surface is directly relevant to the design of steel tube members under combined loading. The yield surface defines the maximum load combinations that the member can sustain before yielding, which is critical for ensuring structural safety and serviceability. The simplified method allows engineers to quickly assess the capacity of CFST members under various load combinations, facilitating the design of efficient and safe structures.

Engineering Practice and Standards Analysis

The plastic yield surface method is applicable to the design of CFST columns, beams, and arch ribs under combined axial and bending loads. The relevant standards include GB 50935 (Code for Design of Concrete-Filled Steel Tubular Structures), which provides design formulas for CFST members but does not explicitly address the plastic yield surface. The proposed method can be used in conjunction with these standards to provide a more comprehensive assessment of member capacity.

The parametric analysis conducted in the study examined the influence of several key parameters on the plastic yield surface, including the steel tube thickness ratio, the concrete strength, the steel yield strength, and the section dimensions. These parameters are directly related to the fabrication and welding of the steel tube members, and their influence on the yield surface should be considered in the design of CFST structures.

In practice, the simplified method can be implemented in structural analysis software to facilitate the plastic hinge analysis of CFST structures. The method's rapid calculation capability makes it suitable for parametric studies and optimization, where multiple design configurations need to be evaluated.

Study Insights and Reflections

This paper represents a valuable contribution to the practical design of CFST structures. The simplified method for determining the plastic yield surface bridges the gap between the accuracy of the fiber model method and the practicality required for engineering design. For steel pipe engineers, the study highlights the importance of understanding the interaction between axial force and bending moment in CFST members, and the need for efficient methods to assess member capacity under combined loading. The parametric analysis provides insight into the factors that influence the yield surface, guiding the design of steel tube sections that provide optimal strength and ductility. This study serves as a reference for engineers seeking to apply plastic hinge analysis to CFST structures in a practical and efficient manner.