Theoretical Analysis of Local Buckling in Axially Compressed Concrete-Filled Steel Tube Columns

Overview of the Literature

The paper by Li Bin, Zhao Hongtie, and Huang Mingkui, published in the Journal of Xi'an University of Architecture and Technology in 2004, presents a theoretical analysis of the local buckling behavior of axially compressed concrete-filled steel tube (CFST) columns. The study is based on experimental investigations of 12 CFST columns with different constraint coefficients, and it analyzes the relationship between failure modes and the constraint coefficient. The authors conclude that the constraint coefficient is the primary factor determining the failure mode of CFST short columns and discuss the optimal constraint coefficient.

Theoretical Framework for Local Buckling

Local buckling is a critical failure mode in CFST columns, particularly for those with slender steel tubes. When the steel tube wall thickness is insufficient relative to the diameter, the tube may buckle locally under the combined action of axial compression and lateral confinement pressure from the expanding concrete core. This local buckling can lead to a sudden loss of load-carrying capacity and a brittle failure mode.

The theoretical analysis of local buckling in CFST columns involves several key considerations. The steel tube is subjected to biaxial stress states: axial compression from the applied load and hoop tension from the lateral expansion of the concrete core. The interaction between these stresses determines the buckling behavior of the steel tube wall.

The local buckling stress can be estimated using classical shell buckling theory, modified for the specific boundary conditions of the CFST column. The steel tube wall is modeled as a cylindrical shell subjected to axial compression and external radial pressure from the concrete core. The critical buckling stress depends on the geometry of the tube, the material properties of the steel, and the boundary conditions at the ends of the column.

The constraint coefficient ξ is defined as the ratio of the confining pressure to the concrete compressive strength, and it is a key parameter in the design of CFST columns. A higher constraint coefficient indicates a stronger confinement effect, which enhances the ductility and energy dissipation capacity of the column.

Experimental Investigation and Failure Mode Analysis

The authors conducted experiments on 12 CFST columns with varying constraint coefficients. The columns were tested under axial compression to failure, and the failure modes were carefully observed and recorded. The constraint coefficient was varied by changing the steel tube dimensions, concrete strength, and column slenderness ratio.

The experimental results revealed three primary failure modes:

The constraint coefficient was found to be the primary factor determining the failure mode. Columns with high constraint coefficients exhibited concrete crushing failure, which is a more ductile and predictable failure mode. Columns with low constraint coefficients exhibited local buckling failure, which is more brittle and less predictable.

The optimal constraint coefficient was identified as approximately 0.8 to 1.2, within which range the columns exhibited a balanced failure mode with good ductility and energy dissipation capacity. Below this range, the columns were prone to brittle local buckling failure, while above this range, the columns may be over-constrained, leading to inefficient use of materials.

Design Implications and Practical Recommendations

Based on the experimental and theoretical analysis, the authors provide several practical recommendations for the design of CFST columns. The constraint coefficient should be carefully controlled to ensure ductile failure behavior and adequate energy dissipation capacity. The steel tube dimensions should be selected to achieve a constraint coefficient within the optimal range of 0.8 to 1.2.

The following design guidelines are recommended:

For seismic applications, the constraint coefficient should be at the higher end of the optimal range to ensure adequate ductility and energy dissipation capacity. The steel tube should be designed to yield before the concrete fails, providing a ductile failure mechanism that can absorb seismic energy.

Critical Reflections and Study Insights

This paper provides valuable insights into the local buckling behavior of CFST columns and the role of the constraint coefficient in determining failure modes. The experimental investigation of 12 columns with varying constraint coefficients provides a solid empirical basis for the theoretical analysis and design recommendations.

One important insight is the direct relationship between the constraint coefficient and the failure mode. This relationship is intuitive and physically meaningful, and it provides a clear design criterion for engineers. The constraint coefficient serves as a simple and effective indicator of the expected failure behavior, allowing engineers to quickly assess the design adequacy of CFST columns.

The identification of an optimal constraint coefficient range is particularly useful for practical design. Engineers can use this range as a target when selecting steel tube dimensions and concrete strength, ensuring that the columns will exhibit ductile failure behavior under axial compression.

However, it is important to note that the constraint coefficient is not the only factor influencing the failure mode. Other factors such as the steel tube material properties, the concrete aggregate type, and the loading rate can also affect the failure behavior. Engineers should consider these factors in addition to the constraint coefficient when designing CFST columns for critical applications.

In summary, this literature provides a comprehensive analysis of local buckling in CFST columns, offering practical design guidelines based on the constraint coefficient that can be readily applied in engineering practice.