Zhuojin Pipe Fitting Co., LtdResidual Bearing Capacity of Hollow Sandwich Square Steel Tube Recycled Concrete Columns After Fire
Literature Overview
The study by Zhang Yuzhuo, Lv Xuetao, and Li Xiaoting (2019), published in Journal of Architecture and Civil Engineering, Vol. 36, No. 1, investigates the post-fire residual mechanical performance of hollow sandwich square steel tube recycled concrete columns. This composite structural system combines the advantages of steel tube confinement, recycled aggregate concrete, and hollow sandwich geometry to achieve enhanced structural performance while promoting sustainable construction practices. The research was funded by the National Natural Science Foundation of China (51208246, 51808352) and the Liaoning Provincial Department of Education (LJYL033), underscoring its academic and practical significance.
Structural Configuration and Finite Element Modeling
The hollow sandwich square steel tube recycled concrete column consists of a square steel tube with a hollow cavity, filled with recycled aggregate concrete. The hollow sandwich design reduces material consumption and self-weight while maintaining structural integrity. The researchers employed ABAQUS finite element software to simulate the behavior of these columns under ISO 834 standard fire conditions, followed by mechanical loading to assess residual capacity.
The finite element model incorporated the following key features:
| Modeling Aspect | Description |
|---|---|
| Steel tube | Bilinear isotropic hardening material model with temperature-dependent properties |
| Recycled concrete | Concrete damaged plasticity model with temperature-dependent strength degradation |
| Hollow cavity | Air gap providing thermal insulation effect |
| Fire loading | ISO 834 standard fire curve applied to exposed surfaces |
| Contact | Penalty contact between steel tube and concrete |
The temperature-dependent material properties were calibrated based on established literature values for structural steel and recycled aggregate concrete, ensuring the model accurately represents the physical behavior of the composite column.
Parametric Study and Key Results
The authors conducted an extensive parametric study examining the influence of seven key parameters on the post-fire residual bearing capacity:
| Parameter | Effect on Residual Bearing Capacity |
|---|---|
| Steel grade | Increases with steel strength |
| Concrete strength | Increases with concrete strength |
| Slenderness ratio | Decreases with increasing slenderness |
| Fire duration | Decreases with fire duration; sharp drop beyond 60 min |
| Hollow ratio | Decreases with increasing hollow ratio |
| Recycled aggregate replacement ratio | Increases then decreases with replacement ratio |
| Eccentricity ratio | Decreases with increasing eccentricity |
The most critical finding is that when the fire duration exceeds 60 minutes, the residual bearing capacity drops precipitously. This threshold represents a critical design consideration, as most structural fire resistance ratings are specified in terms of duration. Beyond 60 minutes, the steel tube undergoes significant strength degradation, and the confinement effect is substantially reduced.
The hollow ratio and recycled aggregate replacement ratio showed relatively minor effects on residual capacity. The hollow ratio causes a gradual decrease in capacity due to reduced concrete volume, while the recycled aggregate replacement ratio exhibits a non-monotonic behavior, with an optimal replacement ratio that balances material sustainability and structural performance.
Simplified Design Formula
Based on the extensive numerical results, the authors derived a simplified calculation formula for the post-fire residual bearing capacity of hollow sandwich square steel tube recycled concrete columns. The formula accounts for the temperature-dependent degradation of both steel and concrete, the geometric effects of the hollow section, and the influence of loading eccentricity. The validation of the formula against the numerical results demonstrated good accuracy, making it suitable for practical design applications.
Engineering Practice and Design Considerations
For engineers designing fire-resistant composite columns, the following considerations emerge from this study:
- Fire resistance duration: The 60-minute threshold should be treated as a critical design parameter. For structures requiring fire resistance beyond 60 minutes, additional protective measures such as fireproof coating or intumescent paint should be considered.
- Material selection: Higher-grade steel and concrete contribute directly to residual capacity, but the cost-benefit ratio must be evaluated for each application.
- Hollow ratio optimization: While the hollow ratio has a relatively minor effect on residual capacity, it should be optimized in conjunction with other design parameters to achieve the best overall structural performance.
- Recycled aggregate usage: The non-monotonic effect of recycled aggregate replacement ratio suggests that there exists an optimal replacement level, typically in the range of 30-50%, beyond which structural performance begins to degrade.
Study Insights and Implications
This research contributes significantly to the field of sustainable structural engineering by demonstrating that recycled aggregate concrete can be effectively used in composite columns without compromising post-fire performance. The hollow sandwich design offers additional advantages in terms of thermal insulation and material efficiency. The derived simplified formula provides a practical tool for engineers to incorporate post-fire residual capacity into their design calculations. However, experimental validation of the numerical results is essential before widespread adoption, particularly regarding the long-term durability of recycled aggregate concrete under cyclic thermal loading. The study opens important avenues for further research into the fire behavior of other composite structural systems incorporating recycled materials.