Axial Compression Performance of Circular-End CFST Medium-Long Columns

Literature Overview

This 2021 study by Ren Zhigang, Zhang Ming, Liu Chuang, and Wang Dandan from Wuhan University of Technology was published in "Building Structure" (Volume 51, Issue 14, pages 62-68). The research was supported by the National Natural Science Foundation of China (Grant No. 51778512). The study investigates the axial compression performance of circular-end concrete-filled steel tube (CFST) medium-long columns, considering the effects of slenderness ratio, steel ratio, and height-to-width ratio through six test specimens and finite element parametric analysis.

Structural Configuration and Design Parameters

The circular-end CFST column features a circular cross-section with a specific end configuration that distinguishes it from conventional CFST columns. The circular cross-section is advantageous for uniform confinement distribution, while the medium-long column classification indicates that both local buckling and global buckling effects must be considered in the design.

Parameter	Description	Variation Range
Slenderness ratio	Column length to equivalent radius	Multiple values
Steel ratio	Steel area to total cross-sectional area	Multiple values
Height-to-width ratio	Column height to cross-sectional dimension	Multiple values
Number of specimens	6 total	Parametric study
Loading type	Axial compression, monotonic	Standard test
Failure mode	Elastic-plastic instability	Observed in all specimens

Loading Stages and Failure Behavior

The experimental results clearly identify three distinct loading stages: the elastic stage, the yielding stage, and the failure stage. The failure mode was consistently identified as elastic-plastic instability failure, which is characteristic of medium-long columns where both material yielding and geometric instability contribute to the ultimate failure.

This progressive failure behavior provides valuable information for design and code calibration. The clear identification of yielding as a distinct stage enables engineers to establish appropriate deformation limits for serviceability and ultimate limit state design.

Parametric Effects on Bearing Capacity

The parametric analysis reveals the following relationships:

Bearing capacity is positively correlated with steel ratio - higher steel content increases the load-bearing capacity of the composite section.
Bearing capacity is negatively correlated with slenderness ratio - longer, more slender columns have reduced capacity due to increased susceptibility to buckling.
For specimens with larger height-to-width ratios, the steel ratio has a more pronounced effect on bearing capacity.

This last observation is particularly significant for practical design. It suggests that in slender CFST columns, increasing the steel ratio is a more effective strategy for improving capacity than other geometric modifications. This finding has direct implications for material selection and cross-section optimization in tall building design.

Finite Element Analysis and Simplified Formula

The researchers used ABAQUS finite element software for parametric analysis and developed a simplified formula for predicting the axial compression bearing capacity of circular-end CFST medium-long columns. The calculated results showed good agreement with experimental results, validating the formula for practical engineering application.

The finite element approach allows for investigation of a wider parameter range than experimental testing alone, providing design guidance for configurations that may not be practical to test physically. The simplified formula derived from this analysis offers engineers a practical tool for capacity estimation during preliminary design stages.

Engineering Practice and Manufacturing Considerations

From a steel pipe manufacturing perspective, the circular-end CFST column requires high-quality circular hollow sections (CHS) with tight dimensional tolerances. The circular cross-section demands precise rolling or forming processes to ensure uniform wall thickness and concentricity. Any out-of-roundness or eccentricity in the tube geometry would reduce the effective confinement and compromise the predicted bearing capacity.

The steel ratio parameter directly relates to the wall thickness selection, which must balance structural performance with manufacturing feasibility and cost. Thicker walls provide higher steel ratio but increase material cost and may require more robust welding procedures for end connections. The slenderness ratio considerations influence the column length and thus the fabrication and erection logistics.

Study Insights and Conclusions

This research provides valuable experimental and analytical data for the design of circular-end CFST medium-long columns. The identification of three distinct loading stages and the elastic-plastic instability failure mode offers clear guidance for performance-based design. The parametric relationships established between steel ratio, slenderness ratio, and bearing capacity provide practical design optimization criteria. The development of a simplified prediction formula, validated against both experimental and finite element results, offers a useful tool for engineers. Future work should extend to combined loading conditions and seismic performance evaluation to enable comprehensive design recommendations for practical applications in buildings and infrastructure.