Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Rectangular CFST Columns with Built-In Special-Shaped Stirrups
Literature Overview
The research by Kang Lan, Chen Xuan, and Hong Shutao (South China University of Technology, 2024) investigates the axial compression mechanical properties of rectangular concrete-filled steel tube (CFST) columns with built-in special-shaped stirrups. Funded by the National Natural Science Foundation of China (grant 52178286) and the Guangdong Provincial Key Laboratory of Modern Civil Engineering Technology (grant 2021B1212040003), this study addresses two critical limitations of conventional rectangular CFST columns: inconsistent constraint between long and short sides, and insufficient confinement of the core concrete.
Experimental Program
The study involves a comprehensive experimental program with 15 specimens:
| Specimen Type | Quantity | Description |
|---|---|---|
| Built-in special-shaped stirrup CFST columns | 11 | Various parameter combinations |
| Built-in racetrack-shaped stirrup CFST columns | 2 | Comparison specimens |
| Conventional rectangular CFST columns | 2 | Baseline specimens |
The parameters investigated include:
- Bundle spacing of the special-shaped stirrups.
- Steel tube wall thickness.
- Concrete strength grade.
- Stirrup spacing.
- Stirrup diameter.
- Built-in steel ratio.
Key Findings and Technical Analysis
The study demonstrates that, under constant total steel usage, reducing the steel tube wall thickness and incorporating the saved steel into built-in special-shaped stirrups effectively improves both the axial compression capacity and ductility of the column. This finding has significant implications for structural design optimization.
The axial compression behavior of the built-in special-shaped stirrup CFST columns is divided into four stages:
| Stage | Description | Stress-Strain Characteristic |
|---|---|---|
| Elastic stage | Linear elastic behavior of all components | Linear increase in load |
| Elastic-plastic stage | Yielding of steel tube and stirrups | Reduced stiffness |
| Plastic strengthening stage | Confinement effect enhances concrete strength | Continued load increase |
| Descending stage | Failure of concrete core and steel components | Load decrease |
The built-in special-shaped stirrup CFST columns exhibit a more pronounced plastic strengthening stage compared to conventional rectangular CFST columns, indicating enhanced ductility and energy dissipation capacity.
Confinement Mechanism Analysis
The confinement mechanism of the built-in special-shaped stirrups operates through several mechanisms:
- Lateral confinement: The stirrups provide lateral restraint to the concrete core, increasing the triaxial compressive strength of the concrete.
- Shear transfer: The stirrups facilitate shear transfer between the steel tube and the concrete core, improving composite action.
- Crack control: The stirrups control the propagation of diagonal cracks in the concrete core, delaying failure.
- Buckling prevention: The stirrups prevent local buckling of the steel tube walls under high compressive stresses.
The confinement effectiveness is quantified using the confined concrete stress-strain model, which relates the lateral confining stress to the enhancement of concrete compressive strength and ductility.
Proposed Axial Compression Capacity Formula
Based on the experimental and parametric analysis results, the authors derive an axial compression capacity formula using existing confined concrete constitutive models. The formula accounts for:
- The contribution of the steel tube.
- The contribution of the confined concrete core.
- The contribution of the built-in stirrups.
- The interaction between these components.
The formula is validated against the experimental results, demonstrating good agreement between predicted and measured capacities.
Welding and Fabrication Considerations
The fabrication of rectangular CFST columns with built-in special-shaped stirrups involves several critical welding operations:
- Steel tube fabrication: The rectangular steel tube is typically fabricated from flat steel plates welded together using submerged arc welding (SAW) or gas metal arc welding (GMAW). The weld quality directly affects the tube's load-bearing capacity.
- Stirrup fabrication: The special-shaped stirrups are fabricated from deformed steel bars, with bends formed at appropriate radii to avoid material damage.
- Stirrup-to-tube connection: The stirrups are connected to the steel tube through welding or mechanical anchoring. The connection must transfer the confinement forces effectively without compromising the tube's structural integrity.
- Concrete pouring: The concrete is poured into the steel tube around the built-in stirrups, requiring careful vibration to ensure proper compaction and bond development.
Quality Control Requirements
For the successful implementation of this structural system, comprehensive quality control is essential:
| Inspection Item | Method | Acceptance Criteria |
|---|---|---|
| Steel tube dimensions | Measurement | Within tolerance per GB 51248 |
| Steel tube weld quality | UT/MT | No cracks, lack of fusion per ISO 5817 |
| Stirrup geometry | Visual inspection | Correct shape and dimensions |
| Concrete strength | Cube testing | Meets specified grade |
| Concrete fill quality | UT or radiography | No voids or segregation |
| Stirrup anchorage | Pull-out testing | Adequate bond strength |
Engineering Practice Integration
The built-in special-shaped stirrup CFST column system offers several advantages for practical applications:
- Improved ductility for seismic-resistant design.
- Optimized steel usage through redistribution from tube walls to internal confinement.
- Enhanced confinement of the concrete core, particularly in rectangular sections where corner regions are less effectively confined by the tube alone.
- Potential for standardized fabrication and field assembly.
However, the system also introduces complexity in fabrication and inspection. The internal stirrups are not directly accessible for visual inspection after concrete pouring, requiring advanced non-destructive testing methods such as phased array ultrasonic testing (PAUT) or ground-penetrating radar (GPR) to verify stirrup placement and concrete quality.
Key Questions and Reflections
The study provides valuable experimental data but raises several questions for practical implementation:
- How does the system perform under cyclic loading conditions, which are critical for seismic design?
- What is the effect of concrete curing conditions on the bond between the stirrups and the concrete core?
- How can the internal stirrup placement be verified in the field without destructive testing?
- What are the long-term durability implications of the embedded stirrups, particularly regarding corrosion protection?
The parametric analysis provides guidance for design optimization, but the interaction between multiple parameters (such as stirrup spacing and steel tube wall thickness) requires further investigation to establish clear design guidelines.
Study Insights and Implications
This research presents an innovative approach to enhancing the axial compression performance of rectangular CFST columns through the strategic use of built-in special-shaped stirrups. The finding that steel redistribution from tube walls to internal confinement improves both capacity and ductility offers a practical design optimization strategy. For engineers involved in the design and construction of CFST structures, this study provides valuable experimental data and analytical tools. However, practical implementation requires careful attention to fabrication quality, particularly the welding of internal connections and the verification of concrete fill quality. Future research should address cyclic loading behavior, durability, and field verification methods to facilitate the widespread adoption of this enhanced CFST column system.