Axial Compression Bearing Capacity and Capacity Composition of Circular Concrete-Filled Steel Tube Columns

Literature Overview

This paper by Chen Likang and Huang Yuan, published in Progress in Steel Building Structures (Vol. 26, Issue 6, 2024, pp. 94–104), presents a comprehensive study on the axial compression bearing capacity and capacity composition of circular concrete-filled steel tube (CCFST) columns. The research was supported by the Hunan Provincial Natural Science Foundation (Grant 2020JJ2003). The authors employed ABAQUS software with a VUMAT subroutine to implement a new concrete material constitutive model, developed a three-dimensional finite element model that captures the interaction between the steel tube and concrete, and derived a mathematical model for the peak bearing capacity under axial compression.

Advanced Finite Element Modeling Approach

The study employs a sophisticated finite element modeling approach that distinguishes itself from conventional analysis methods through several key features:

VUMAT Subroutine Implementation

The authors implemented a new concrete material constitutive model through ABAQUS's VUMAT (User-Defined Material Subroutine for Explicit Dynamics) subroutine. This approach allows for:

• Accurate representation of concrete behavior under triaxial stress states
• Capture of the confinement effect induced by the steel tube
• Modeling of the progressive degradation of concrete strength under compression
• Representation of the interaction between steel tube deformation and concrete stress state

Three-Dimensional Interaction Model

The developed three-dimensional finite element model captures the mechanical interaction between the steel tube and concrete core through:

Model Validation

The finite element model was validated against experimental data by comparing:

• Load-displacement curves of the entire CCFST column
• Load-deformation curves of the steel tube component
• Load-deformation curves of the concrete component
• Lateral deformation curves

The good agreement between numerical and experimental results confirms the accuracy and reliability of the developed model.

Parametric Study Results

The authors conducted a comprehensive parametric study to investigate the influence of key parameters on the bearing capacity and capacity composition of CCFST columns:

Concrete Strength

• Higher concrete strength increases the overall bearing capacity of the CCFST column.
• The concrete contribution to total capacity increases with concrete strength, but the relative contribution of the steel tube may decrease.
• The confinement effect becomes more significant at higher concrete strength levels.

Steel Tube Yield Strength

• Higher steel tube yield strength increases the overall bearing capacity.
• The steel tube contribution to total capacity increases with yield strength.
• The confinement effectiveness improves with higher steel tube strength.

Diameter-to-Thickness Ratio (D/t)

• The D/t ratio significantly affects the local buckling behavior of the steel tube.
• Higher D/t ratios reduce the confinement effectiveness due to increased tube flexibility.
• The concrete contribution to capacity may increase relative to the steel tube contribution at higher D/t ratios, as the tube becomes less effective at confining the concrete.

Confinement Coefficient

• The confinement coefficient, defined as the ratio of steel tube area to concrete area, is a critical parameter.
• Higher confinement coefficients increase the confinement effectiveness and overall bearing capacity.
• The interaction between steel and concrete becomes more pronounced at higher confinement coefficients.

Mathematical Model Development

Based on the finite element analysis results, the authors derived a mathematical model for the peak bearing capacity of CCFST columns under axial compression. The model introduces two key coefficients:

1. Steel tube hoop stress ratio coefficient: Evaluates the stress state of the steel tube and its contribution to the overall capacity.
2. Concrete bearing capacity contribution coefficient: Quantifies the contribution of the confined concrete to the overall bearing capacity.

These coefficients provide a practical tool for engineers to assess the individual contributions of the steel tube and concrete to the total bearing capacity, which is essential for rational design and material optimization.

Comparison with Existing Standards and Research

The proposed calculation formula was compared with existing design codes and research findings:

The proposed model demonstrates good calculation accuracy and provides the unique advantage of evaluating the individual contributions of the steel tube and concrete to the overall bearing capacity.

Implications for Steel Pipe Manufacturing and Selection

The research findings have direct implications for steel pipe manufacturing and product selection for CCFST applications:

Tube Specification Optimization

• The diameter-to-thickness ratio is a critical parameter that affects both the steel tube's direct load-bearing capacity and its confinement effectiveness.
• Manufacturing engineers must balance wall thickness (which affects material cost and manufacturability) against the structural performance requirements.
• Tighter dimensional tolerances on wall thickness may be required for CCFST applications to ensure consistent confinement effectiveness.

Material Grade Selection

• The steel tube yield strength directly influences the confinement effectiveness and overall column capacity.
• Higher-grade steels (e.g., Q390, Q420) provide better confinement but may require more careful welding procedure qualification.
• The selection of steel grade must consider the interaction between yield strength, ductility, and weldability.

Welding Quality Requirements

• The longitudinal weld in the steel tube (if applicable) must maintain full cross-sectional integrity to ensure effective confinement.
• Weld defects can compromise the confinement effect and reduce the effective bearing capacity.
• Non-destructive testing protocols should be tailored to CCFST applications, with particular attention to weld quality verification.

Surface Quality and Dimensional Accuracy

• The internal surface quality of the steel tube affects the bond between the concrete core and the steel tube.
• Dimensional accuracy of the tube (circularity, straightness, wall thickness uniformity) is critical for consistent confinement effectiveness.
• Manufacturing engineers must implement quality control measures to ensure that tube specifications meet CCFST application requirements.

Study Insights and Reflections

This research represents a significant advancement in the understanding of CCFST column behavior through the development of a sophisticated finite element model that captures the steel-concrete interaction. The introduction of the steel tube hoop stress ratio coefficient and the concrete bearing capacity contribution coefficient provides engineers with practical tools for evaluating the individual contributions of each component to the overall bearing capacity.

From a manufacturing engineering perspective, the parametric study results highlight the importance of precise control of tube geometry and material properties. The diameter-to-thickness ratio, which is a key manufacturing parameter, has a significant influence on the confinement effectiveness and overall structural performance. This finding underscores the need for tight manufacturing tolerances and quality control measures in steel pipe production for CCFST applications.

The mathematical model developed in this study offers a more detailed understanding of capacity composition compared to existing design codes. This level of detail enables more rational design decisions, including the optimization of tube specifications to achieve target capacity while minimizing material cost. For steel pipe manufacturers, this translates into the potential for value-added products with optimized geometry and material specifications tailored to CCFST applications.

The validation of the finite element model against experimental data provides confidence in the accuracy of the proposed approach. The ability to predict not only the overall bearing capacity but also the individual contributions of the steel tube and concrete is a significant advancement that supports more informed engineering decisions.

Summary

This comprehensive study on the axial compression bearing capacity and capacity composition of circular CCFST columns provides valuable insights through advanced finite element modeling and mathematical model development. The introduction of the steel tube hoop stress ratio coefficient and concrete bearing capacity contribution coefficient offers engineers practical tools for evaluating the individual contributions of each component to the overall structural performance. For steel pipe manufacturing engineers, the research underscores the critical importance of precise tube geometry, appropriate material grade selection, and high-quality welding in ensuring the structural performance of CCFST columns. The proposed mathematical model represents a significant advancement over existing design codes, providing the level of detail necessary for rational design and material optimization in CCFST applications.