Zhuojin Pipe Fitting Co., LtdConstitutive Model of Recycled Concrete-Filled Square Steel Tube Columns Under Cyclic Loading
Literature Overview
The paper by Zhang Xianggang and colleagues from Henan Polytechnic University and Guangxi University, published in the Journal of Disaster Prevention and Mitigation Engineering in 2016, addresses a critical gap in seismic design of recycled concrete-filled square steel tube (RCFST) columns. With the global push toward sustainable construction, recycled coarse aggregate replacement has become increasingly common, yet the inelastic constitutive behavior of such hybrid members under low-cycle reversed loading remains poorly characterized. The authors designed six specimens with varying recycled coarse aggregate replacement rates and axial compression ratios, subjected them to quasi-static cyclic tests, and developed a three-linear constitutive model based on characteristic points extracted from hysteresis curves.
Core Technical Findings
The experimental program varied two key parameters: recycled coarse aggregate replacement rate (which governs the quality of the infill concrete) and axial compression ratio (which controls the ductility and energy dissipation capacity). The failure modes observed were consistent with conventional steel tube concrete columns, with localized buckling occurring at the bottom of the square steel tube and crushing of the recycled concrete at the base. This is significant because it suggests that the recycled aggregate does not fundamentally alter the failure mechanism, provided the replacement rate is kept within reasonable bounds.
The hysteresis loops exhibited satisfactory fullness and stability, indicating that RCFST columns retain adequate energy dissipation capacity even with recycled aggregate. This is an important finding for seismic design, as the ductility and cumulative energy dissipation are primary performance indicators for earthquake-resistant structures.
Constitutive Model Development
The authors employed three special processing methods to extract characteristic points from the hysteresis curves: fixed-point pointing, displacement amplitude bearing capacity sudden drop detection, and model softening point identification. These methods were used to determine the relative yield point, relative peak point, and relative damage point, which together define the three-linear force-displacement constitutive model.
| Model Characteristic Point | Identification Method | Engineering Significance |
|---|---|---|
| Relative Yield Point | Fixed-point pointing on hysteresis curve | Onset of inelastic behavior; defines elastic limit |
| Relative Peak Point | Displacement amplitude bearing capacity sudden drop | Maximum lateral force capacity; defines ultimate strength |
| Relative Damage Point | Model softening point analysis | Onset of significant stiffness degradation |
The three-linear model was validated against measured hysteresis curves and showed good agreement. The authors recommend its application in elasto-plastic seismic response analysis of this new type of composite structure.
Engineering Practice Implications
From a steel pipe manufacturing perspective, the square steel tube used in these columns must meet strict geometric tolerances, particularly regarding wall thickness uniformity and corner radius consistency. Square steel tubes are typically produced by roll-forming or welding processes, and any variation in corner geometry directly affects the local buckling behavior observed in the tests. The bottom buckling failure mode highlights the importance of the tube's initial out-of-straightness and residual stresses from the manufacturing process.
For welding engineers, the connection details between the square steel tube and the foundation or adjacent structural elements are critical. The localized buckling at the column base suggests that the boundary conditions and restraint provided by the connection significantly influence the failure mode. In practice, this means that welded or bolted base connections must be designed to provide adequate rotational restraint without creating stress concentrations that could initiate premature buckling.
Key Questions and Reflections
A key question raised by this study is the optimal recycled coarse aggregate replacement rate that balances sustainability goals with structural performance. The study suggests that moderate replacement rates do not fundamentally alter failure modes, but the precise threshold beyond which performance degradation becomes unacceptable requires further investigation. Additionally, the interaction between recycled aggregate quality (surface texture, water absorption, mechanical interlock) and the constitutive model parameters warrants deeper exploration.
The three-linear constitutive model, while practical for seismic analysis, may oversimplify the actual inelastic behavior, particularly the pinching effect and stiffness degradation observed in real hysteresis curves. Future work should consider more sophisticated models that capture the evolution of unloading stiffness and the accumulation of damage under repeated cycling.
Study Insights and Outlook
This study provides a valuable foundation for the seismic design of recycled concrete-filled steel tube columns, contributing both experimental data and a practical constitutive model. The findings support the viability of recycled aggregate in structural applications, which aligns with global sustainability objectives. However, the research is limited to quasi-static cyclic loading, and the dynamic behavior under earthquake excitation may differ due to strain rate effects and the interaction between the steel tube and recycled concrete at high loading rates. Future research should extend to dynamic testing and consider the long-term durability implications of recycled aggregate exposure to environmental cycles, including freeze-thaw and chloride ingress, which are particularly relevant for steel pipe members in aggressive environments.