Finite Element Analysis of Axial Compression Behavior of Circular Stainless Steel Tube Concrete Members

Literature Overview and Research Context

The paper by Liao Feiyu, published in the Journal of Fujian Agriculture and Forestry University (Natural Science Edition) in 2009 (Vol. 38, No. 6, pp. 659-662), addresses the axial compression mechanical behavior of circular stainless steel tube concrete (SSTC) members through finite element analysis. The research was funded by the Fujian Province Young Scientific and Technological Talent Innovation Fund (2008F3007), the Fujian Provincial Department of Education (JA08066), and the Fujian Agriculture and Forestry University Young Teacher Fund (08A2). Stainless steel tube concrete members offer significant advantages in corrosive environments, making them particularly suitable for marine infrastructure, chemical plant piping, and other applications where carbon steel would require costly protective coatings or cathodic protection.

Methodology and Finite Element Model Development

The study establishes a finite element model that accounts for both geometric nonlinearity (large displacements and large rotations) and material nonlinearity (constitutive relationships for stainless steel and concrete). Three critical modeling decisions were made:

1. Material constitutive models were defined for both the stainless steel tube and the core concrete, capturing the distinct stress-strain behaviors of each material.
2. Appropriate element types were selected to accurately represent the three-dimensional stress state within the composite member.
3. An interface contact model was developed to simulate the interaction between the stainless steel tube and the core concrete, including the confinement effect and potential debonding.

Material Constitutive Models

The constitutive model for stainless steel typically follows a Ramberg-Osgood type relationship, which captures the smooth transition from elastic to plastic behavior without a sharp yield point. This is in contrast to carbon steel, which exhibits a more pronounced yield plateau. The stress-strain relationship can be expressed as:

For the core concrete, a confined concrete model was adopted that accounts for the lateral confinement provided by the stainless steel tube. The confinement effect increases both the compressive strength and the ductility of the concrete, with the degree of confinement dependent on the diameter-to-thickness ratio (D/t) of the stainless steel tube and the material properties of the tube steel.

Interface Contact Modeling

The interface between the stainless steel tube and the core concrete is modeled using a penalty-based contact algorithm or a Lagrange multiplier method. The contact model must capture the following behaviors:

• Radial expansion of the concrete core under axial compression, which increases the lateral pressure on the inner surface of the steel tube.
• Potential micro-cracking and debonding at the steel-concrete interface under high strain levels.
• The frictional resistance that prevents relative sliding between the steel tube inner surface and the concrete core.

Results and Comparative Analysis

The finite element results were validated against experimental test data, and the study reports that the overall agreement between calculated and experimental results is good. This validation is essential for establishing confidence in the numerical model before using it for parametric studies.

The study compares the load-deformation relationship curves of stainless steel tube concrete members with those of conventional carbon steel tube concrete members. The key differences observed include:

The higher post-peak ductility of stainless steel tube concrete members is particularly significant for seismic applications, as it indicates greater capacity for inelastic deformation and energy dissipation. The gradual post-peak descent in the load-deformation curve suggests that the member retains a significant portion of its load-carrying capacity even after substantial plastic deformation, which is a desirable characteristic for progressive collapse resistance.

Engineering Practice Integration and Quality Considerations

From a fabrication standpoint, the welding of stainless steel tube concrete members presents unique challenges compared to carbon steel counterparts. Stainless steels such as AISI 304, 316, and 321 have lower thermal conductivity and higher thermal expansion coefficients than carbon steels, which increases the risk of distortion and residual stress during welding. The following welding considerations are critical:

• Welding process selection: GTAW (Tungsten Inert Gas Welding) is preferred for root passes due to its ability to produce clean, oxide-free welds. FCAW (Flux-Cored Arc Welding) with stainless flux-cored wire is suitable for filler and cap passes in thicker sections.
• Heat input control: Low heat input is essential to minimize the formation of chromium carbide precipitates at the grain boundaries in the HAZ, which can lead to intergranular corrosion. Heat input should be kept below 1.5 kJ/mm where possible.
• Filler metal selection: Filler metals with appropriate chromium and nickel content must be selected to maintain the corrosion resistance of the weld. For example, ER308L is commonly used for AISI 304 base metal, while ER316L is used for AISI 316.
• Post-weld treatment: Solution annealing at 1050-1100 degrees Celsius followed by rapid quenching may be necessary for critical welds to restore full corrosion resistance.

The concrete pouring process into stainless steel tubes also requires careful attention. The stainless steel tube interior surface is typically smoother than carbon steel, which may reduce the mechanical interlock between the concrete and the steel tube. Surface roughening or the use of mechanical keys at strategic locations can improve the bond strength. The concrete mix design should incorporate appropriate water-reducing admixtures to achieve the required workability for pumping into the confined tube space without excessive segregation.

Study Insights and Implications

The finite element model developed in this study provides a valuable tool for parametric investigation of SSTC member behavior, enabling the exploration of design variables such as D/t ratio, concrete strength, stainless steel grade, and member slenderness ratio without the cost and time associated with physical testing. The good agreement between numerical and experimental results validates the model for use in design optimization studies.

However, I observe that the study does not explicitly address the long-term behavior of SSTC members under sustained loading, including the effects of concrete creep and shrinkage on the stress redistribution between the steel tube and the concrete core. In service, these time-dependent effects can significantly influence the structural performance, particularly for members subjected to long-term axial compression. The interaction between concrete creep and the confinement pressure from the stainless steel tube is a complex coupled phenomenon that deserves further investigation.

Additionally, the corrosion performance of the stainless steel tube in the presence of chlorides from the concrete or from the external environment is not addressed. While stainless steels offer superior corrosion resistance compared to carbon steels, pitting and crevice corrosion can still occur under certain conditions, particularly at welds and at the steel-concrete interface where the local chemistry may differ from the bulk environment. Long-term exposure testing and the application of corrosion allowance in design are recommended.

This study makes a meaningful contribution to the understanding of SSTC member behavior under axial compression and provides a validated numerical framework that can be extended to more complex loading conditions and structural configurations in future research.