Zhuojin Pipe Fitting Co., LtdCalculation Method for Compression-Bending Capacity of Square CFST Columns Strengthened with Outer Square Steel Tube and Interlayer Concrete
Literature Overview
This study proposes a calculation method for the compression-bending capacity of square concrete-filled steel tube (CFST) columns that have been externally strengthened with an additional square steel tube and an interlayer concrete layer. This strengthening approach is commonly applied to retrofit existing CFST columns in buildings and industrial structures that require increased load-bearing capacity due to functional changes, code updates, or damage repair.
Strengthening Method and Structural Configuration
The strengthening method involves welding an outer square steel tube to the existing CFST column and filling the annular space between the two tubes with interlayer concrete. This creates a double-skin CFST column configuration with enhanced load-bearing capacity, improved ductility, and increased resistance to local buckling.
Material and Geometric Parameters
| Parameter | Existing Column | Strengthening Layer | Combined System |
|---|---|---|---|
| Steel tube grade | Q235/Q345 | Q345/Q390 | Composite |
| Concrete grade | C30–C50 | C50–C80 | Composite |
| Outer tube thickness | t₁ (original) | t₂ (added) | t₁ + t₂ |
| Interlayer concrete | None | f_c2 | Confinement |
| Connection method | N/A | Welded/flanged | Integral |
Theoretical Analysis and Calculation Method
The study develops a unified calculation method based on the principle of superposition and the interaction between the existing column and the strengthening layer. The compression-bending capacity is determined by considering:
- Axial load capacity: The total axial capacity is the sum of the steel tube contributions, the concrete core capacity, and the interlayer concrete capacity, modified by the confinement effect.
- Bending moment capacity: The bending capacity accounts for the plastic neutral axis position, the strain distribution across the section, and the interaction between axial load and bending moment.
- Interaction curve: The M-N interaction curve is generated by varying the eccentricity of the axial load and computing the corresponding moment capacity.
Key Equations and Parameters
| Parameter | Symbol | Description |
|---|---|---|
| Axial load | N | Applied compressive force |
| Bending moment | M | Applied bending moment |
| Eccentricity | e = M/N | Load eccentricity |
| Confinement ratio | ξ = (A_s f_y) / (A_c f_c) | Steel-to-concrete ratio |
| Confinement coefficient | φ | Enhancement factor |
| Plastic neutral axis | x | Depth of neutral axis |
| Section modulus | W | Resisting moment |
The calculation method incorporates several important assumptions:
- Perfect bond between the existing column and the strengthening layer.
- Plane sections remain plane after deformation.
- The interlayer concrete provides additional confinement to the inner concrete core.
- The outer steel tube and inner steel tube act compositely after strengthening.
Comparison with Existing Methods
The study compares the proposed method with several existing calculation approaches, including the ACI 318 method, the GB 50936 method, and the Eurocode 4 method. The comparison reveals that existing methods tend to be conservative for the double-skin CFST configuration because they do not fully account for the enhanced confinement effect provided by the interlayer concrete.
| Method | Axial Capacity | Moment Capacity | Conservatism |
|---|---|---|---|
| ACI 318 | Overestimates | Overestimates | ~20% |
| GB 50936 | Slightly overestimates | Slightly overestimates | ~10–15% |
| Eurocode 4 | Underestimates | Underestimates | ~15–25% |
| Proposed method | Accurate | Accurate | ~5% |
Experimental Validation
The study validates the proposed calculation method through a series of tests on strengthened square CFST columns. The test specimens varied in geometric dimensions, material grades, and loading conditions. The results demonstrate that the proposed method predicts the compression-bending capacity with an average error of less than 10%, which is well within acceptable engineering accuracy.
Test Results Summary
| Specimen | N/N_pred | M/M_pred | Failure Mode |
|---|---|---|---|
| S1 | 0.95 | 0.97 | Local buckling |
| S2 | 0.98 | 0.96 | Concrete crushing |
| S3 | 0.92 | 0.94 | Combined failure |
| S4 | 0.96 | 0.98 | Steel yielding |
| S5 | 0.94 | 0.95 | Concrete crushing |
Engineering Application and Practical Considerations
The proposed calculation method has direct applications in structural retrofitting projects. When strengthening existing CFST columns, engineers must consider:
- Construction feasibility: The welding of the outer tube to the existing column must be performed carefully to avoid excessive heat input that could damage the existing concrete core.
- Quality control: The interlayer concrete must be placed with adequate compaction to ensure full contact with both steel tubes.
- Compatibility: The strengthening layer should be designed to accommodate differential settlement and thermal expansion between the existing column and the new layer.
- Seismic performance: The strengthened column should maintain adequate ductility and energy dissipation capacity under seismic loading.
Study Insights and Reflections
This research addresses a practical and important engineering need for the retrofitting of existing CFST structures. The proposed calculation method provides a reliable tool for engineers to design strengthening schemes with confidence. The key insight is that the double-skin CFST configuration offers a synergistic effect where the interlayer concrete not only contributes to the load-bearing capacity but also enhances the confinement of the inner concrete core, creating a more efficient structural system.
The study also highlights the importance of considering the construction sequence and quality control in strengthening projects. The effectiveness of the strengthening layer depends critically on the bond quality between the new and existing components. Poor workmanship during the welding or concrete placement can significantly reduce the actual capacity below the predicted values.
This literature provides a valuable contribution to the field of structural retrofitting and offers a practical calculation method that can be readily applied in engineering practice. The method's accuracy and simplicity make it suitable for inclusion in future design codes and standards.