Zhuojin Pipe Fitting Co., LtdOpenSees-Based Hysteresis Analysis of Double Steel Tube High-Strength Concrete Columns
Literature Overview
The paper by Li Ningbo, Qian Jiaru, Ji Xiaodong, Zhao Zuozhou, and Ye Lieping (Tsinghua University, 2014) presents a numerical investigation using the OpenSees (Open System for Earthquake Engineering Simulation) platform to analyze the seismic performance of double steel tube high-strength concrete (CFDT) columns. The study employs fiber-based modeling with appropriate constitutive relationships to capture the complex interaction between the outer square steel tube, inner circular steel tube, and high-strength concrete core.
Numerical Modeling Approach
Element and Fiber Section Configuration
| Component | Material Model | Key Parameters | Justification |
|---|---|---|---|
| Steel tube (outer square) | Steel02 (bilinear) | Fy, Es, b, d, Rt, fl | Captures Bauschinger effect |
| Steel tube (inner circular) | Steel02 (bilinear) | Same as above | Consistent steel behavior |
| Concrete (outer zone) | Concrete02 (unconfined) | f'c, eps_0, eps_cu | Standard confined concrete model |
| Concrete (inner zone) | Susantha model | Confinement parameters | Captures tube-concrete interaction |
| Column element | Fiber section + displacement-based | Geometric nonlinearity | Captures P-Delta effects |
Constitutive Model Selection Rationale
The selection of the Susantha model for the concrete confined between the steel tubes is particularly noteworthy. This model accounts for the progressive confinement pressure developed by the steel tubes as the concrete undergoes lateral expansion under axial compression. The bilinear Steel02 model with kinematic hardening captures the cyclic behavior of steel tubes including the Bauschinger effect and strain hardening.
Parametric Analysis Results
Influence of Axial Load Ratio
| Axial Load Ratio | Ductility Index | Energy Dissipation | Peak Load Capacity |
|---|---|---|---|
| 0.2 | High | Excellent | Moderate increase |
| 0.4 | Moderate-high | Good | Significant increase |
| 0.6 | Moderate | Adequate | Continued increase |
| 0.8 | Low | Limited | Slight increase |
Increasing the axial load ratio reduces ductility significantly, as the compressive stress state limits the development of plastic deformation mechanisms. However, the vertical and horizontal bearing capacity initially increases with axial load due to the confinement effect, until a critical ratio is exceeded.
Influence of Square Tube Wall Thickness (Width-to-Thickness Ratio)
Reducing the width-to-thickness ratio (increasing wall thickness) of the outer square steel tube enhances the horizontal bearing capacity through improved local buckling resistance. The confinement effectiveness increases proportionally with wall thickness, leading to improved concrete confinement and overall column performance.
Influence of Circular Tube Diameter and Wall Thickness
Increasing the inner circular steel tube diameter and wall thickness contributes positively to both vertical and horizontal bearing capacity. The enhanced confinement from a thicker circular tube provides more effective restraint of the high-strength concrete core, improving the column's energy dissipation capability under cyclic loading.
Model Validation and Engineering Relevance
The calculated hysteresis curves show good agreement with experimental results, particularly in the elastic and early inelastic range. Discrepancies increase after yield, which is typical for fiber-based models that do not fully capture local buckling and steel-concrete bond slip phenomena. Despite these limitations, the model provides valuable insight into the relative influence of geometric parameters on seismic performance.
Practical Implications for Steel Tube Selection
| Application Requirement | Recommended Configuration | Design Consideration |
|---|---|---|
| High ductility demand | Lower axial ratio, thinner square tube | Balance with capacity requirements |
| High strength demand | Thicker circular tube, higher f'c | Watch for brittle failure mode |
| Seismic application | Moderate axial ratio (0.3-0.4) | Optimize energy dissipation |
| Economic optimization | Standard wall thickness ratios | Minimize material usage |
Study Insights and Reflections
This research demonstrates the power of fiber-based numerical modeling in understanding complex structural behavior that would be prohibitively expensive to investigate experimentally. The parametric study provides design engineers with quantitative guidance on optimizing double steel tube column configurations for seismic applications.
From a steel tube manufacturing perspective, the findings emphasize the importance of wall thickness uniformity and geometric precision in double tube column applications. The interaction between the two steel tubes and the concrete core creates complex stress states that are sensitive to manufacturing tolerances. Engineers should ensure that both tubes meet the dimensional specifications per GB/T 8162 or equivalent standards, with particular attention to the circular tube's roundness and the square tube's flatness.
The OpenSees modeling approach presented here provides a transferable methodology that can be adapted for other composite column configurations, making this research valuable beyond its specific application to double steel tube columns.