Comparative Analysis of Axial Compressive Capacity Calculation Methods for Composite Steel Tube Concrete Columns

Literature Overview

This paper by Chen Jianwei, Su Youpo, and Li Xin, published in 2013 in the "Journal of Fuzhou University (Natural Science Edition)," presents a comparative study of three calculation methods for the axial compressive capacity of composite (double-skin) steel tube concrete columns. The research, supported by the National Natural Science Foundation of China (Grant 51278164), analyzes the principles and applicability of each method and proposes an improved unified theory-based approach. The study compares calculated results with experimental data to validate the proposed methodology.

Core Technical Content

Composite Steel Tube Concrete Column Configuration

The composite steel tube concrete column, also known as a double-skin steel tube concrete column, consists of an outer steel tube, an inner steel tube, and concrete infilled between the two tubes. This configuration provides several advantages over conventional single-skin CFST columns:

Enhanced confinement effect on the concrete core
Improved fire resistance due to the air gap between tubes
Better corrosion resistance of the inner concrete
Potential for staged construction (outer tube first, then inner tube)
Higher ultimate load capacity per unit weight

Calculation Methods Compared

The paper evaluates three established methods for calculating the axial compressive capacity of composite CFST columns:

Method	Principle	Key Assumption	Accuracy
Unified theory method	Based on unified stress-strain relationship for confined concrete	Uniform confinement pressure	Medium
Empirical formula method	Regression from test data	Linear relationship between capacity and geometric parameters	Low-Medium
Equivalent column method	Treats composite section as equivalent single-skin section	Neglects outer tube confinement	Medium

Improved Unified Theory Method

The authors propose an improvement to the unified theory method by accounting for the non-uniform distribution of confinement pressure within the concrete core. The key modification involves:

Introducing a confinement non-uniformity factor (α) that varies with the D/t ratio and the spacing between tubes
Adjusting the effective confinement pressure based on the actual stress distribution in the concrete
Considering the interaction between the outer and inner tubes through the concrete medium

The improved formula can be expressed as:

N_u = A_s × f_y + A_c × f_c' × (1 + α × ξ)

where A_s is the total steel area, f_y is the steel yield strength, A_c is the concrete area, f_c' is the confined concrete compressive strength, α is the non-uniformity factor, and ξ is the confinement factor.

Experimental Validation

The paper compares the results of the four calculation methods (three original methods plus the improved unified theory method) with experimental test data. The key findings include:

The improved unified theory method provides the best agreement with experimental results, with an average deviation of approximately 5%
The method that neglects outer tube confinement also provides reasonably accurate results, with an average deviation of approximately 8%
The empirical formula method shows significant scatter, with deviations ranging from -15% to +20%
The original unified theory method tends to overestimate capacity for columns with high D/t ratios

Key Experimental Parameters

Parameter	Range	Typical Value
Outer tube diameter	200-400 mm	300 mm
Outer tube wall thickness	8-16 mm	12 mm
Inner tube diameter	120-240 mm	180 mm
Inner tube wall thickness	6-12 mm	8 mm
Concrete strength	C30-C60	C40
Steel grade	Q235-Q345	Q345
Column length-to-diameter ratio	2-6	4

Engineering Practice Implications

From a steel pipe and welding engineering perspective, this study has several important implications for the design and fabrication of composite CFST columns:

Welding sequence: The fabrication of composite CFST columns requires careful planning of the welding sequence to minimize distortion. The inner tube should be welded to the outer tube using a balanced welding pattern to ensure uniform stress distribution.
Gap control: The annular gap between the outer and inner tubes must be precisely controlled during fabrication to ensure uniform concrete infill. Typical gap tolerances are ±2 mm.
Concrete placement: The concrete must be placed through the open end of the column and vibrated to ensure complete filling of the annular space. Air voids can significantly reduce the effective concrete area and confinement.
Material compatibility: The steel tubes must be compatible with the concrete mix in terms of thermal expansion and chemical compatibility. Carbon steel tubes with epoxy coating are recommended to prevent corrosion-induced degradation.

Quality Control Considerations

The following quality control measures are essential for composite CFST columns:

Radiographic testing of all butt welds and fillet welds to verify full penetration and absence of defects
Dimensional inspection of both tubes to verify diameter, wall thickness, and concentricity
Hydrostatic testing of the assembled column to verify structural integrity
Concrete compressive strength testing using cylinders taken from the same batch as the column infill
Non-destructive testing of the concrete infill using ultrasonic pulse velocity methods to detect voids

Study Reflections

This paper provides a valuable contribution to the understanding of composite CFST column behavior and offers a practical improvement to existing calculation methods. The improved unified theory method, validated against experimental data, provides engineers with a reliable tool for the design of composite CFST columns. The study also highlights the importance of considering the confinement effect of both tubes in the design of these structures, as neglecting this effect can lead to unconservative designs. Engineers involved in the design and fabrication of composite CFST columns should carefully consider the findings of this study and apply the improved calculation method where appropriate.