Numerical Analysis of Concrete-Filled Steel Tube Members with Initial Stress Using ANSYS

Literature Overview

This paper by Wang Weisheng, Wu Caoliang, and He Xi, published in Subgrade Engineering (2007, Issue 3, pp. 99-101), presents a finite element approach using ANSYS to calculate the load-bearing capacity of concrete-filled steel tube (CFST) members that experience initial stresses. The authors are affiliated with Chongqing Jiaotong University, China Communications Construction Third Navigation Engineering Company, and Xiamen Municipal Construction Development Corporation, representing a collaboration between academic research and practical engineering.

Technical Approach and Methodology

The study establishes a numerical analysis model in ANSYS that accounts for the influence of initial stress on the full-process load-strain relationship of CFST members under both axial compression and eccentric compression. The use of ANSYS Programming Language (APDL) enables automated parametric studies and efficient model generation.

Model Development Strategy

Modeling Aspect	Approach
Geometry	Shell elements for steel tube, solid elements for concrete core
Material behavior	Bilinear and multilinear stress-strain models
Initial stress	Applied through initial stress field or equivalent nodal loads
Contact interface	Frictional contact between steel tube and concrete
Convergence	Arc-length method for nonlinear analysis
Output	Full load-strain curves for comparison with tests

Key Technical Points

Initial Stress Considerations

Initial stresses in CFST members can arise from:

Construction sequence effects (staged loading during erection)
Thermal gradients during fabrication or service
Residual stresses from the steel tube manufacturing process (particularly relevant for HFW welded tubes)
Preloading in structural systems where members are not simultaneously loaded

The paper demonstrates that neglecting initial stress can lead to significant errors in predicted load-bearing capacity, particularly for members with higher slenderness ratios or those subjected to eccentric loading.

Validation Against Experimental Data

Through instance calculations and comparison with experimental results, the authors confirm that the ANSYS-based approach for calculating initial-stress CFST member capacity is effective and reliable. The numerical model captures the essential nonlinear behavior including:

Progressive yielding of the steel tube
Concrete confinement and crushing behavior
Interaction between steel and concrete through the contact interface
Post-peak softening behavior

Engineering Practice Integration

For steel pipe manufacturers and fabricators, the consideration of initial stress has several practical implications:

Residual stress management: The welding and forming processes used to manufacture steel tubes introduce residual stresses that constitute an initial stress state. Understanding how these affect composite member behavior helps in specifying appropriate heat treatment or stress-relief procedures.
Fabrication tolerances: Initial geometric imperfections (ovality, wall thickness variation) create localized initial stress concentrations that the numerical model can capture when properly represented.
Construction sequencing: The APDL-based parametric approach allows engineers to simulate different construction sequences and identify the most favorable erection procedures that minimize adverse initial stress effects.

Study Insights and Reflections

This paper represents a practical application of finite element methods to a specific engineering problem that is often overlooked in design practice. The availability of APDL for parametric studies makes this approach particularly valuable for design optimization and sensitivity analysis. The validation against experimental data provides confidence in the numerical predictions. For the steel pipe industry, this work highlights the importance of documenting residual stress states in manufactured tubes and communicating this information to structural engineers for accurate design analysis. The methodology can be extended to other composite systems and loading conditions, making it a versatile tool for advanced structural analysis. The collaboration between academia and industry represented in this work exemplifies the productive integration of research capabilities with practical engineering needs.