Mechanical Performance of Square Concrete-Filled Steel Tube Axially Compressed Short Columns

Literature Overview

This study by Ding Faxing and colleagues from Central South University, published in the Journal of Shenzhen University (Science and Engineering) in 2014, investigates the mechanical behavior of square concrete-filled steel tube (SCFST) axially compressed short columns through a combination of physical testing and ABAQUS finite element analysis. Supported by the National Natural Science Foundation (No. 50908230) and the National Science and Technology Support Program (No. 2011BAJ09B02), the research develops a practical bearing capacity formula validated against 98 sets of existing experimental data.

Research Methodology

Experimental Program

Three SCFST short column specimens were tested under axial compression to obtain baseline experimental data for validating the finite element model. The specimens represent typical square CFST cross-sections used in structural applications, with square steel tubes filled with concrete to form composite columns.

Finite Element Modeling Approach

The validated ABAQUS three-dimensional solid finite element model was used to conduct extensive parametric analysis. The modeling approach incorporated:

Realistic material constitutive models for both steel and concrete components
Appropriate contact definitions between the steel tube inner surface and concrete core
Geometric nonlinearity to capture large deformation effects
Material nonlinearity to represent plastic behavior of both materials

Key Technical Findings

Stress Distribution Analysis

The finite element analysis revealed distinct stress patterns in the core concrete at ultimate limit state. The authors identified two zones:

Zone	Description	Stress State	Confinement Level
Strengthened Zone	Concrete uniformly confined by steel tube	High triaxial compression	Full confinement
Non-Strengthened Zone	Concrete not effectively confined by steel tube	Near uniaxial compression	Minimal confinement

The area ratio between these two zones was determined through systematic finite element analysis, providing a quantitative basis for the proposed bearing capacity formula.

Average Steel Stress Ratio

A critical parameter identified through the parametric study is the ratio of average longitudinal stress in the square steel tube to its yield strength at ultimate limit state. This ratio is not a constant but depends on geometric parameters such as the cross-sectional dimensions and the concrete strength grade.

Proposed Bearing Capacity Formula

Based on the equilibrium limit theory and the finite element-derived parameters, the authors proposed a new practical formula for SCFST short column bearing capacity. Validation against 98 sets of existing experimental data demonstrated that this formula provides the best agreement with test results compared to existing formulas in the literature.

Comparison with Existing Formulas

Formula Source	Average Error	Applicability	Key Assumption
Existing Formula A	Larger deviation	General SCFST	Simplified confinement model
Existing Formula B	Moderate deviation	Specific range	Uniform confinement assumption
Proposed Formula	Best agreement	Broad parameter range	Two-zone confinement model

Engineering Significance for Steel Pipe Practice

The two-zone confinement model is particularly insightful from a steel pipe engineering perspective. In square CFST columns, the corners of the steel tube provide less effective confinement to the concrete than the mid-span regions of the tube walls. This is because the flat walls of a square tube deflect more under internal pressure than curved sections, leading to non-uniform confinement pressure distribution.

For steel pipe fabrication and specification in SCFST applications:

The wall thickness of square steel tubes must be sufficient to resist the internal expansive pressure from confined concrete, particularly at the mid-span of each flat wall where deflection is greatest.
Local buckling resistance of the steel tube walls becomes a critical design consideration, as premature local buckling would reduce the confinement effectiveness.
The proposed formula's accuracy across 98 datasets provides confidence for using it in design, but engineers should verify that the specific steel grade and concrete strength fall within the validated parameter range.

Reflections on the Two-Zone Model

The two-zone confinement concept represents a significant advancement over traditional models that assume uniform confinement throughout the concrete core. In practice, this means that the actual bearing capacity of SCFST columns is lower than what uniform confinement models would predict, and the proposed formula's conservative accuracy is more appropriate for design purposes. For steel pipe engineers, this reinforces the importance of wall thickness adequacy—not merely for carrying direct loads, but for maintaining effective confinement over the entire concrete core area. The research demonstrates the power of combining experimental validation with extensive finite element parametric analysis to develop practical design formulas that are both accurate and broadly applicable.