Finite Element Simulation and Mechanism Analysis of Axial Compression Behavior of Steel Pipe Self-Consolidating Concrete Members

Literature Overview

The paper by Wang Qingli et al., published in Journal of Shenyang Jianzhu University (2011, Vol. 27, No. 6, pp. 1043–1052), presents a finite element analysis of the static behavior of steel pipe self-consolidating concrete (SCC) members under axial compression. The research was supported by the National Natural Science Foundation of China (10972144) and contributed to the development of the Liaoning Provincial Standard DB21/T1746-2009 for steel pipe concrete structures. The authors established validated FEA models to investigate the stress and strain distributions in both the steel pipe and the SCC core, and to understand the interaction mechanism between the two materials.

Background and Motivation

Self-consolidating concrete (SCC) is a highly flowable concrete that can fill formwork under its own weight without the need for vibration. When used as the core material in steel pipe concrete members, SCC offers several advantages:

• Elimination of vibration, which is particularly beneficial for members with closely spaced reinforcement or complex geometries.
• Improved workability, ensuring complete filling of the steel pipe without voids or honeycombing.
• Enhanced durability due to the dense microstructure resulting from the self-compacting process.

However, the mechanical behavior of steel pipe SCC members under axial compression has not been extensively studied, particularly regarding the interaction between the steel pipe and the SCC core. This study aims to fill that gap through both experimental testing and finite element simulation.

Finite Element Modeling

The FEA models were established in ABAQUS and validated against experimental test results. The following modeling approaches were adopted:

The concrete constitutive model was calibrated to reproduce the uniaxial stress-strain behavior of the SCC, including the ascending branch, peak stress, and descending branch. The steel constitutive model included elastic-plastic behavior with strain hardening.

Key Simulation Results

Stress and Strain Distributions

The FEA results revealed important patterns in the stress and strain distributions within the steel pipe SCC members:

• Circular sections: The interaction force between the steel pipe and the SCC core is distributed uniformly around the circumference. This uniform distribution is a direct consequence of the geometric symmetry of the circular section.
• Square sections: The interaction force is maximum at the corner regions of the square section. This is because the corners experience higher stress concentrations due to the geometric discontinuity, and the SCC core exerts greater lateral pressure on the steel pipe at these locations.
• Axial distribution: The interaction force between the steel pipe and the SCC core decreases with increasing distance from the mid-section. This is because the boundary conditions at the ends of the member constrain the lateral expansion of the SCC core, reducing the confinement effect near the ends.

Effect of Bond Strength

The study found that the bond strength at the steel-SCC interface has a relatively minor effect on both the load-carrying capacity and the interaction force. This is an important practical finding because it suggests that the performance of steel pipe SCC members is not highly sensitive to the quality of the interface bond, which simplifies construction quality control.

Comparison with Ordinary Concrete

A significant conclusion of the study is that for steel pipe SCC members with concrete compressive strength up to 96 MPa, the load-carrying capacity can be calculated using the existing design formulas for steel pipe ordinary concrete members. The static behavior of steel pipe SCC members is essentially the same as that of steel pipe ordinary concrete members.

This finding has important implications for engineering practice:

• Design code compatibility: Existing design codes and standards for steel pipe concrete members can be applied to SCC-filled members without modification.
• Material substitution: SCC can be used as a direct replacement for ordinary concrete in steel pipe concrete members without requiring changes to the structural design.
• Construction flexibility: The use of SCC provides greater construction flexibility (no vibration required) without compromising structural performance.

Welding and Fabrication Considerations

The fabrication of steel pipe SCC members requires attention to the following welding and construction aspects:

• Pipe straightness and ovality: The steel pipe should be inspected for straightness and ovality before concrete filling. Excessive ovality can lead to non-uniform concrete cover thickness, which may affect the confinement effect.
• End preparation: The ends of the steel pipe should be prepared for connection to the structural framing with appropriate weld details. The welds should be designed to accommodate the expected deformations under axial compression.
• SCC placement: The SCC should be placed in layers to avoid segregation and to ensure uniform filling. The placement rate should be controlled to prevent excessive hydrostatic pressure on the steel pipe.
• Curing: After SCC placement, the member should be cured under appropriate conditions to ensure proper strength development. The steel pipe can act as a formwork, but the ends should be sealed to prevent moisture loss.

Study Insights and Reflections

This study provides valuable insights into the behavior of steel pipe SCC members and demonstrates that SCC is a viable alternative to ordinary concrete in steel pipe concrete applications. The finding that the static behavior is essentially the same as that of ordinary concrete members is particularly important for engineering practice, as it means that existing design methodologies can be applied without modification.

The FEA model developed in this study is a valuable tool for further investigations, such as the behavior of steel pipe SCC members under combined loading (axial compression with bending), under cyclic loading, or under fire conditions. The model can also be extended to study the effect of different SCC mix designs, including the use of supplementary cementitious materials or fibers.

The contribution of this research to the development of the Liaoning Provincial Standard DB21/T1746-2009 is noteworthy, as it demonstrates the direct link between academic research and practical code development.