Nonlinear Analysis of Circular Steel Tube Steel Reinforced Recycled Concrete Composite Columns Based on OpenSees

Literature Overview

The paper by Wang Defa, Meng Zexiang, Ma Hui, and Guo Tingting, published in Journal of Xi'an University of Technology (2020, Vol. 36, No. 1, pp. 88-94), presents a nonlinear numerical analysis of circular steel tube steel reinforced recycled concrete (RCSC) composite columns using the OpenSees finite element software. This research is particularly relevant in the context of sustainable construction, where the use of recycled aggregates in concrete is gaining increasing attention as a means to reduce environmental impact and promote circular economy principles.

Research Background and Significance

Recycled concrete, which incorporates recycled coarse aggregates from demolished concrete structures, offers significant environmental benefits compared to conventional concrete. However, the use of recycled aggregates introduces uncertainties in mechanical properties due to:

• Residual mortar attached to recycled aggregates, which reduces aggregate strength
• Higher water absorption of recycled aggregates
• Potential microcracks in recycled aggregates
• Variability in aggregate properties from different demolition sources

The composite column configuration studied—circular steel tube with internal steel reinforcement and recycled concrete infill—represents a hybrid structural system that combines:

• The confinement effect of the circular steel tube
• The load-bearing contribution of internal steel reinforcement (H-section or similar)
• The compressive strength of recycled concrete

Numerical Modeling Approach

OpenSees Fiber Model

The authors employed a fiber model based on the flexibility method to represent the composite column cross-section. The cross-section was discretized into multiple material zones, each assigned appropriate material properties:

Element Type

The column was modeled using nonlinear beam-column elements from the frame element library in OpenSees. These elements incorporate geometric nonlinearity and material nonlinearity, allowing for accurate simulation of the column behavior under axial compression.

Model Validation

The numerical model was validated against experimental data from 11 test specimens. The comparison showed that the relative error in predicted load capacity was generally within 10%, indicating good accuracy of the fiber model approach.

Parameter Study and Key Findings

The authors conducted a parametric study to investigate the influence of several key parameters on the axial compression performance of the RCSC columns:

Parameter Effects

Detailed Analysis of Each Parameter

Diameter-to-thickness ratio (d/t): A higher d/t ratio results in a thinner steel tube wall, which reduces the confinement effect on the concrete core and increases the susceptibility to local buckling. This leads to a reduction in load capacity. The optimal d/t ratio should be selected to balance structural efficiency with stability requirements.

Steel reinforcement ratio: The internal steel reinforcement contributes directly to the axial load capacity. A higher reinforcement ratio increases the load capacity but may also affect the ductility of the column. The reinforcement ratio should be optimized to achieve the desired balance between strength and ductility.

Slenderness ratio: The slenderness ratio (L/i, where L is the column length and i is the radius of gyration) significantly affects the load capacity through the Euler buckling mechanism. Higher slenderness ratios lead to earlier buckling and reduced load capacity. This is a fundamental design consideration for all column structures.

Recycled aggregate replacement rate: Surprisingly, the recycled aggregate replacement rate had minimal influence on the load capacity. This finding is encouraging from a sustainability perspective, as it suggests that recycled concrete can be used in composite columns without significant structural penalty. However, this finding should be interpreted with caution, as the study may not have covered the full range of replacement rates (0-100%).

Engineering Practice Implications

Design Guidelines

Based on the research findings, the following design guidelines are recommended:

1. Diameter-to-thickness ratio: Limit d/t to values that prevent local buckling, typically d/t ≤ 100-120 for carbon steel tubes, in accordance with GB 51246 or similar standards.
2. Steel reinforcement ratio: Select the reinforcement ratio based on the required load capacity and ductility. Typical values range from 2-10% of the cross-sectional area.
3. Slenderness ratio: Design for slenderness ratios that provide adequate safety margin against buckling. For composite columns, slenderness ratios of 30-60 are typically acceptable.
4. Recycled aggregate replacement rate: While the research suggests minimal influence on load capacity, engineers should still verify the mechanical properties of recycled concrete through testing and ensure compliance with relevant standards (GB/T 25177, EN 12620).

Quality Control Considerations

For the construction of RCSC columns, the following quality control measures are recommended:

Study Insights and Reflections

This research makes a valuable contribution to the advancement of sustainable construction practices by demonstrating that recycled concrete can be effectively used in composite column structures without significant compromise in structural performance. The finding that recycled aggregate replacement rate has minimal influence on load capacity is particularly encouraging for the construction industry's transition towards more sustainable practices.

However, several limitations and areas for further investigation should be noted:

1. Long-term durability: The research focuses on short-term compression performance, but the long-term durability of recycled concrete in composite columns, particularly in terms of carbonation, chloride ingress, and freeze-thaw resistance, warrants further investigation.
2. Seismic performance: While the research addresses axial compression, the seismic behavior of RCSC columns, which is critical for structures in earthquake-prone regions, requires separate investigation through cyclic loading tests and corresponding numerical analyses.
3. Fire resistance: The fire performance of recycled concrete-filled steel tube columns is an important consideration that has not been addressed in this study.
4. Corrosion protection: The internal steel reinforcement in the composite column is exposed to the concrete environment. While the concrete provides some protection, the presence of recycled aggregates with higher porosity may increase the risk of corrosion. Engineers should consider appropriate corrosion protection measures.
5. Standardization: The widespread adoption of recycled concrete in structural applications requires the development and harmonization of relevant standards. Engineers should advocate for and contribute to the development of standards that address the specific requirements of recycled concrete in composite structures.

In conclusion, the nonlinear analysis of circular steel tube steel reinforced recycled concrete composite columns using OpenSees provides a reliable numerical tool for the design and evaluation of these sustainable structural systems. The research supports the use of recycled concrete in composite columns and offers valuable insights into the key design parameters that influence structural performance.