Zhuojin Pipe Fitting Co., LtdExperimental Study on Pure Bending Members of Hollow Sandwich Steel Tube Concrete with Tie Bars
Literature Overview
The research by Ding Faxing, Lu Deren, Zhang Tao, and Pan Zhicheng from Central South University, published in the Journal of Earthquake Engineering and Engineering Vibration in 2020 (Vol. 40, No. 4, pp. 25-34), presents an innovative structural solution for hollow sandwich steel tube concrete (HSC) members. The key innovation is the welding of bidirectional tension tie bars between the inner and outer steel tubes to enhance the mechanical performance of the composite cross-section. This work is supported by the National Key R&D Program of China (2017YFC0703404) and the Hunan Province Distinguished Young Scholars Fund (2019JJ20029).
Structural Concept and Design Rationale
Hollow Sandwich Steel Tube Concrete Configuration
The hollow sandwich steel tube concrete (HSC) member consists of an inner steel tube, an outer steel tube, and a concrete layer sandwiched between them. This configuration offers several advantages over conventional filled steel tube concrete (FSTC):
| Feature | FSTC | HSC | HSC with Tie Bars |
|---|---|---|---|
| Concrete core | Solid | Sandwich layer | Sandwich layer |
| Inner tube | None | Yes | Yes |
| Tie bars | None | None | Bidirectional |
| Concrete confinement | Full | Partial | Partial + enhanced |
| Local buckling risk | Low | Moderate | Reduced |
| Construction complexity | Low | Moderate | Higher |
The primary challenge with HSC members is the tendency of the inner and outer steel tubes to locally buckle (bulge outward) under compressive loading, which reduces the confinement effectiveness of the steel tubes on the sandwich concrete. The tie bars are introduced to mechanically connect the inner and outer tubes, preventing relative radial displacement and maintaining the integrity of the sandwich concrete layer.
Tie Bar Configuration
The bidirectional tension tie bars are welded at regular intervals between the inner and outer steel tubes. The tie bars are oriented in two orthogonal directions to provide uniform radial constraint in all circumferential directions. The spacing and diameter of the tie bars are critical design parameters that influence the overall performance.
Experimental Program and Results
Specimen Configuration
The experimental program included specimens with and without tie bars for comparative analysis. The specimens were subjected to pure bending loading to evaluate their ultimate bending moment, bending stiffness, and lateral deformation characteristics.
| Parameter | With Tie Bars | Without Tie Bars |
|---|---|---|
| Ultimate bending moment | Higher | Lower |
| Bending stiffness | Higher | Lower |
| Lateral deformation coefficient | Lower | Higher |
| Local buckling of inner tube | Significantly reduced | Pronounced |
| Local buckling of outer tube | Significantly reduced | Moderate |
| Concrete confinement effectiveness | Enhanced | Reduced |
Key Findings
The experimental results demonstrate several important findings:
- Tie bars effectively mitigate local buckling of both the inner and outer steel tubes, maintaining the structural integrity of the sandwich concrete layer throughout the loading process.
- The ultimate bending moment is significantly improved compared to specimens without tie bars, indicating that the tie bars contribute to a more efficient utilization of the composite cross-section.
- The bending stiffness is enhanced due to the improved interaction between the steel tubes and the concrete layer, which reduces the relative slip and radial displacement between components.
- The lateral deformation coefficient is reduced, indicating better control of out-of-plane deformation and improved overall stability.
Finite Element Analysis Validation
A three-dimensional solid finite element model was developed using ABAQUS software, incorporating a reasonable three-axial stress constitutive model for concrete and a steel constitutive model. The model was validated against experimental results and showed good agreement. The finite element analysis further revealed the stress distribution in the inner and outer steel tubes and the sandwich concrete, confirming that the tie bars enhance the confinement effect and promote more rational force distribution among the components.
Stress Distribution Analysis
Effect on Steel Tube Stresses
The finite element analysis shows that the tie bars significantly alter the stress distribution in the steel tubes:
- Without tie bars: The inner tube experiences pronounced local buckling under compression, leading to stress concentration at the buckled regions. The outer tube also shows local buckling tendencies, particularly on the compression side of the bending section.
- With tie bars: The stress distribution in both tubes is more uniform. The tie bars prevent the radial displacement that leads to local buckling, allowing the steel tubes to develop their full compressive and tensile capacity.
Effect on Concrete Stresses
The tie bars strengthen the confinement action of both steel tubes on the sandwich concrete. This enhanced confinement leads to:
- Higher triaxial compressive stress in the concrete on the compression side.
- More uniform stress distribution in the concrete layer.
- Delayed crushing and spalling of the concrete.
- Improved ductility of the overall member.
Engineering Practice Implications
Welding Considerations
The welding of tie bars between the inner and outer tubes presents specific challenges:
| Welding Parameter | Recommendation |
|---|---|
| Welding process | GTAW (TIG) or GMAW (MIG) for precision |
| Weld type | Full-penetration fillet weld |
| Preheating | Required for thick-walled tubes (t > 20 mm) |
| Interpass temperature | Controlled to prevent excessive HAZ hardening |
| Post-weld treatment | Stress relief if residual stress is critical |
| NDT method | UT or MT for weld inspection |
The welds must be designed to transfer the full tension force in the tie bars without premature failure. The weld HAZ must not develop excessive hardness or cracking susceptibility, particularly if the steel tubes are made of higher-grade materials such as Q345 or Q460.
Construction Sequencing
The construction of HSC members with tie bars requires careful sequencing:
- Fabrication of inner and outer tubes with tie bar attachment points.
- Welding of tie bars to both tubes (typically on the inner tube first for accessibility).
- Assembly of the inner tube with tie bars into the outer tube.
- Concrete pouring of the sandwich layer.
- Curing and quality inspection.
The welding sequence should minimize residual stresses and distortion, particularly for large-diameter tubes where thermal distortion can be significant.
Key Questions and Reflections
The study raises several important questions for further investigation:
- What is the optimal spacing of tie bars for different tube diameters and wall thicknesses?
- How does the tie bar configuration affect the performance under seismic (cyclic) loading conditions?
- What are the long-term durability implications of the tie bar welds, particularly in corrosive environments?
- Can the tie bar concept be extended to HSC columns and shear walls?
The interaction between the tie bars and the concrete sandwich layer is complex. The concrete must be properly confined during pouring to ensure full contact with both tubes and the tie bars. Any voids or honeycombing in the concrete layer would reduce the effectiveness of the tie bar system.
Study Insights and Implications
This research demonstrates a practical and effective method for enhancing the performance of hollow sandwich steel tube concrete members. The tie bar concept is straightforward in principle but requires careful attention to welding quality, construction sequencing, and design optimization. The combination of experimental validation and finite element analysis provides a comprehensive understanding of the mechanical behavior and the role of tie bars in the structural system.
For structural engineers and steel fabricators, the key insight is that the mechanical connection between the inner and outer tubes is critical for the performance of HSC members. Without such connection, the sandwich concrete cannot be effectively confined, and the steel tubes cannot develop their full structural capacity. The tie bar solution is a practical engineering response to this fundamental challenge, and the experimental results confirm its effectiveness.
In summary, this study contributes a valuable structural innovation to the field of composite steel-concrete construction, offering a path to improved performance of HSC members through a relatively simple but well-conceived welding detail.