Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Hollow Sandwich Thin-Walled Steel Tube Concrete Short Columns
Literature Overview
The paper by Wang Zhibin and colleagues from the College of Civil Engineering, Fuzhou University, published in Progress in Steel Building Structures (2018, Vol. 20, No. 2, pp. 53-59), presents an experimental and numerical investigation of the axial compression behavior of hollow sandwich thin-walled steel tube concrete short columns with longitudinal stiffening ribs. The study examines the influence of hollow ratio and diameter-to-thickness ratio on the mechanical properties of the composite columns. The research was supported by the National Natural Science Foundation of China (Grant 51708118), Fujian Provincial Natural Science Foundation (2017J01696), and Fuzhou Science and Technology Program (2017-G-101).
Core Technical Content
Specimen Configuration
The study involved 9 axially loaded short column specimens with the following configuration:
| Parameter | Description | Range |
|---|---|---|
| Column type | Hollow sandwich thin-walled SC short column | - |
| Outer tube | Circular steel tube | Φ150-200 mm |
| Inner tube | Circular steel tube (concentric) | Φ100-150 mm |
| Concrete | Filled between outer and inner tubes | C30-C50 |
| Hollow ratio | Inner tube area / Total area | 0.2-0.5 |
| Diameter-to-thickness ratio | D/t of outer tube | 15-30 |
| Longitudinal stiffening ribs | Welded to outer tube | 4-8 ribs |
Experimental Findings
The experimental results demonstrate that hollow sandwich thin-walled SC short columns exhibit:
- High load-bearing capacity: The composite action between steel tubes and concrete provides significant strength enhancement.
- Good ductility: The hollow configuration improves the ductility of the column compared to solid SC columns.
- Effective confinement: The outer steel tube provides confinement to the concrete, particularly at the corners where the longitudinal stiffening ribs are located.
Numerical Simulation
Finite element analysis was performed using ABAQUS, with the following modeling approach:
- Steel tubes: Elastic-plastic constitutive model with von Mises yield criterion.
- Concrete: Modified Mohr-Coulomb model with confinement-dependent strength parameters.
- Steel-concrete interface: Frictional contact with bond-slip behavior.
- Longitudinal stiffening ribs: Shell elements with appropriate thickness and material properties.
The numerical simulation results showed good agreement with experimental measurements, validating the modeling approach and constitutive assumptions.
Mechanism Analysis
The study reveals that:
- Outer tube confinement effect: The confinement provided by the outer steel tube is concentrated at the corners where the longitudinal stiffening ribs are located. This is because the ribs enhance the local buckling resistance of the outer tube, allowing it to maintain its circular shape and provide effective confinement.
- Longitudinal stiffening rib contribution: The ribs significantly improve the load-bearing capacity of the outer steel tube by preventing local buckling and enhancing the composite action between the outer tube and the concrete.
- Hollow ratio effect on ductility: Increasing the hollow ratio improves the ductility of the column by reducing the concrete volume and allowing greater deformation before failure.
Parameter Analysis
The parametric study reveals the following trends:
| Parameter | Effect on Load-Bearing Capacity | Effect on Ductility |
|---|---|---|
| Steel tube yield strength | Increases | Decreases slightly |
| Concrete strength | Increases | Decreases slightly |
| Diameter-to-thickness ratio | Increases (up to limit) | Increases |
| Hollow ratio | Decreases | Increases |
| Number of stiffening ribs | Increases | Increases |
Engineering Practice Integration
Design Formula Development
Based on the experimental and numerical results, the authors proposed a simplified calculation formula for the load-bearing capacity of hollow sandwich thin-walled SC short columns:
N_u = f_s × A_s + f_c' × A_c × (1 + α × k_c × D/t)
Where:
- N_u = Ultimate axial load capacity
- f_s = Yield strength of steel tube
- A_s = Cross-sectional area of steel tubes
- f_c' = Compressive strength of concrete
- A_c = Cross-sectional area of concrete
- α = Empirical coefficient
- k_c = Confinement enhancement factor
- D/t = Diameter-to-thickness ratio of outer tube
Welding and Fabrication Considerations
The fabrication of hollow sandwich thin-walled SC columns with longitudinal stiffening ribs requires careful attention to welding quality:
| Weld Type | Location | Inspection Method | Acceptance Criteria |
|---|---|---|---|
| Butt weld | Outer tube splices | 100% RT | Level I per GB/T 3323 |
| Butt weld | Inner tube splices | 100% RT | Level I per GB/T 3323 |
| Fillet weld | Stiffening rib to outer tube | 100% UT | Level II per GB/T 11345 |
| Fillet weld | Rib end connections | VT + MT | No cracks or porosity |
Quality Control Procedures
The quality control process for hollow sandwich SC columns should include:
- Incoming material inspection: Verify steel tube dimensions, chemical composition, and mechanical properties against ASTM A53, ASTM A106, or GB/T 8163 specifications.
- Pre-weld preparation: Bevel preparation, fit-up, and alignment inspection.
- Welding process monitoring: Welding parameter records, welder qualification verification.
- Post-weld NDT: RT for butt welds, UT for fillet welds, MT or PT for surface inspection.
- Concrete placement verification: UT to detect voids in the annular space between outer and inner tubes.
- Dimensional inspection: Verify column straightness, tube concentricity, and rib spacing.
Application Scenarios
Hollow sandwich thin-walled SC columns are suitable for:
- High-rise building columns: Where high load-bearing capacity and ductility are required.
- Bridge piers: Subject to seismic loading and requiring good energy dissipation.
- Industrial structures: Where lightweight construction with high strength is desired.
- Retrofitting projects: Where existing columns need strength enhancement without significant size increase.
Study Insights and Reflections
The hollow sandwich thin-walled SC column concept represents an innovative approach to improving the ductility of steel tube concrete columns while maintaining high load-bearing capacity. The key insight is that the hollow configuration, combined with longitudinal stiffening ribs, creates a synergistic effect where the outer tube provides effective confinement, the inner tube prevents outward buckling of the concrete, and the ribs enhance the local stability of the outer tube.
The finding that the outer tube confinement effect is concentrated at the corners is particularly interesting from a mechanics perspective. This suggests that the stress distribution in the composite column is non-uniform, with higher confinement pressures at the rib locations. This non-uniformity should be considered in detailed design calculations and may influence the failure mode of the column.
From a welding engineering perspective, the fabrication of these columns requires high-quality welds to ensure the composite action between the steel tubes, concrete, and stiffening ribs. Any weld defect could compromise the load transfer between components and reduce the overall column capacity. Therefore, rigorous welding quality control and NDT are essential for the safe and reliable performance of hollow sandwich thin-walled SC columns.
The simplified calculation formula proposed by the authors provides a practical tool for engineers to estimate the load-bearing capacity of these columns. However, the formula should be validated against a larger database of experimental results before being adopted in design codes. Future research should also investigate the behavior of these columns under combined axial and lateral loading, cyclic loading, and fire conditions to provide a comprehensive understanding of their structural performance.