Axial Compression Mechanical Properties of Special-Shaped Concrete-Filled Steel Tube Short Columns

Literature Overview

This paper by Ren Qingxin, Gao Lei, and Wang Qingli, published in Industrial Construction (2014, Vol. 44, No. 4, pp. 21–26), investigates the axial compression behavior of short columns made from concrete-filled steel tubes with non-circular cross-sections. Five special-shaped cross-sections—triangular, fan-shaped, D-shaped, quarter-circle, and half-circle—were studied through finite element analysis using ABAQUS, validated against experimental data for circular section specimens. The research was funded by the National Natural Science Foundation of China (Grant 51208135), the Liaoning Provincial Bureau of Quality and Technical Supervision (2012DB155B), and the Liaoning Provincial Department of Education (L2011089).

Cross-Sectional Geometry and Finite Element Modeling

The study examines five non-circular cross-section geometries, all designed with equal perimeter length to the reference circular section. This equal-perimeter approach ensures a fair comparison of structural efficiency across different shapes.

The finite element models were developed in ABAQUS using solid elements with appropriate contact definitions between the steel tube and concrete core. The material models incorporated:

• Steel tube: Elastic-plastic constitutive law with von Mises yield criterion and isotropic hardening.
• Concrete: Concrete Damaged Plasticity (CDP) model capturing nonlinear behavior under compression and tension.
• Steel-concrete interface: Frictional contact with appropriate friction coefficients to simulate bond behavior.

Experimental Validation

The finite element results were validated against experimental data from six groups of circular-section CFST short column tests. The validation criteria included:

• Load-strain curves: Comparison of full-range load-strain responses between FE predictions and experimental measurements.
• Failure modes: Qualitative comparison of buckling patterns and failure mechanisms.
• Ultimate loads: Quantitative comparison of peak load values.

The paper reports good agreement between finite element calculations and experimental results, confirming the reliability of the numerical model for predicting the behavior of special-shaped CFST columns.

Parametric Study Results

The parametric analysis investigated the influence of four key parameters on the bearing capacity and deformation characteristics of special-shaped CFST short columns:

Influence of Cross-Section Shape

The cross-section shape significantly affects the structural performance. Among the five tested geometries, the half-circle section exhibited the highest bearing capacity due to its favorable combination of material distribution and confinement effectiveness. The triangular section performed the weakest, primarily due to its low geometric efficiency and poor confinement distribution.

Influence of Steel Tube Wall Thickness (t)

Increasing the wall thickness of the steel tube had a pronounced positive effect on bearing capacity. Thicker tubes provide:

• Greater confinement pressure on the concrete core
• Higher local buckling resistance
• More effective composite action between steel and concrete

Influence of Steel Yield Strength (fy)

The steel yield strength showed a surprisingly limited influence on the overall structural performance. This counterintuitive result can be explained by the fact that in short columns, the failure is governed by concrete crushing and steel tube local buckling rather than steel yielding. Once the steel tube yields, further increases in yield strength do not significantly improve the post-yield confinement capacity.

Influence of Concrete Compressive Strength (fcu)

Higher concrete compressive strength directly improved the bearing capacity, as expected. However, the benefit diminishes at very high strength levels due to the increased brittleness of high-strength concrete, which can lead to more sudden failure modes.

Failure Mode Analysis

All special-shaped CFST short columns exhibited outward bulging failure at approximately the mid-height of the column. The failure mechanism involves:

1. Initial elastic loading: Linear load-strain relationship with uniform stress distribution.
2. Steel tube yielding: Local yielding initiates at the mid-height where bending moments from eccentricities are maximum.
3. Concrete crushing: Progressive concrete crushing develops as the steel tube begins to buckle outward.
4. Local buckling propagation: Buckling waves propagate along the column length, with the most severe deformation at mid-height.
5. Post-peak behavior: Gradual strength degradation as the buckled steel tube loses confinement effectiveness.

Engineering Practice Implications

The findings have several practical implications for structural engineering applications:

• Shape optimization: For applications requiring special-shaped CFST columns (e.g., architectural columns, irregular structural layouts), the half-circle and D-shaped sections offer the best balance of structural efficiency and geometric practicality.
• Material selection: The limited benefit of high-strength steel suggests that cost-effective medium-strength steels (Q345 or Q390) are adequate for most CFST column applications, with resources better allocated to concrete strength optimization.
• Design codes: Current design codes for CFST columns are primarily based on circular and square/rectangular sections. The findings suggest that appropriate modification factors should be developed for special-shaped sections to ensure safe and economical design.
• Manufacturing considerations: Special-shaped steel tubes can be manufactured through plate rolling and welding, but the welding quality and geometric accuracy must be carefully controlled to ensure consistent structural performance.

Key Reflections and Insights

This research addresses an important gap in the structural engineering literature regarding the behavior of non-circular CFST columns. In practice, architectural requirements and site constraints often necessitate non-standard cross-section geometries, yet the available design guidance for such configurations is limited.

The finding that steel yield strength has limited influence on the overall performance is particularly noteworthy. This suggests that the confinement mechanism in CFST columns is governed more by the geometric properties (tube thickness, cross-section shape) and concrete properties than by the steel strength itself. This insight has direct implications for material selection and cost optimization in CFST column design.

The equal-perimeter comparison methodology is a sound approach for evaluating geometric efficiency, but I would note that practical manufacturing constraints—such as minimum wall thickness requirements, welding accessibility, and formwork complexity—may limit the applicability of certain shapes in real engineering projects.

The finite element modeling approach using ABAQUS with appropriate contact and material models provides a reliable tool for predicting the behavior of special-shaped CFST columns. However, the accuracy of the predictions depends heavily on the quality of the input parameters, particularly the steel-concrete interface friction coefficients and the concrete damage model parameters. Sensitivity studies on these parameters would strengthen the confidence in the numerical predictions.

This work contributes valuable data for the development of design guidelines for special-shaped CFST columns and opens the door for further research on the behavior of more complex geometries, including multi-cell configurations and hybrid steel-concrete-steel sandwich sections. Future studies should also investigate the seismic performance of special-shaped CFST columns, as the irregular cross-sections may exhibit different hysteretic behavior compared to conventional circular sections.