Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Square Hollow Sandwich High-Strength Steel Pipe Concrete Columns
Literature Overview and Research Background
This study investigates the axial compression behavior of square hollow sandwich high-strength steel pipe columns filled with high-performance concrete (HPC). The sandwich configuration involves an inner steel tube, an outer steel tube, and a high-strength concrete layer placed between them, creating a composite structural system that leverages the confinement effects of both steel shells. The research addresses a critical gap in the understanding of multi-layer composite columns, which are increasingly adopted in high-rise buildings and long-span structures where conventional single-tube steel-concrete composite columns may not provide sufficient load-bearing capacity or ductility.
From a materials science perspective, the selection of high-strength steel grades (typically Q460 or Q690) combined with HPC (compressive strength exceeding 60 MPa) creates synergistic interactions that warrant careful examination of the interface behavior and stress transfer mechanisms. The sandwich design introduces additional complexity in terms of welding connections, interfacial bonding, and potential for delamination under cyclic or sustained loading.
Core Technical Findings
Geometric and Material Parameters
| Parameter | Typical Range | Remarks |
|---|---|---|
| Outer tube width | 300–600 mm | Square cross-section |
| Wall thickness (outer) | 8–20 mm | High-strength steel Q460/Q690 |
| Inner tube width | 200–400 mm | Concentric arrangement |
| Wall thickness (inner) | 6–14 mm | High-strength steel Q345/Q460 |
| Concrete compressive strength | C60–C100 | High-performance concrete |
| Slenderness ratio | 3–12 | Short to medium columns |
| Concrete cover thickness | 20–40 mm | Interface layer |
The experimental results demonstrate that the sandwich configuration significantly enhances the ultimate load capacity compared to single-tube steel-concrete composite columns of equivalent steel weight. The confinement effect from the outer tube prevents outward expansion of the concrete, while the inner tube provides additional axial load resistance and helps distribute stresses more uniformly across the cross-section.
Load-Displacement Response
The load-displacement curves exhibit three distinct stages: elastic stage, yielding stage, and descending stage. During the elastic stage, the steel tubes and concrete behave independently with minimal interaction. As the load increases beyond the proportional limit, the concrete begins to bear a greater share of the axial force, and the confinement effect becomes more pronounced. The peak load is typically achieved when the concrete reaches its confined compressive strength, which can be 1.5 to 2.0 times the unconfined concrete strength depending on the confinement ratio.
Interface Behavior and Failure Modes
The interfacial bond between the inner steel tube and the HPC is critical to the overall structural performance. Shear keys or corrugated inner surfaces significantly improve the bond strength and prevent premature slippage. The primary failure mode observed in the tests involves local buckling of the outer tube walls, followed by concrete crushing and eventual crushing of the inner tube. The sandwich configuration delays the onset of local buckling compared to single-tube configurations due to the lateral support provided by the concrete layer.
Welding and Manufacturing Considerations
From a welding engineering standpoint, the fabrication of sandwich steel-concrete composite columns presents unique challenges. The connection between the inner tube and the outer tube, typically achieved through welded shear connectors or continuous welds, requires careful control of welding parameters to avoid excessive heat input that could compromise the mechanical properties of the high-strength steel. Preheating temperatures of 100–150°C are recommended for Q460 and Q690 steels to minimize the risk of hydrogen-induced cracking.
| Welding Parameter | Recommended Value | Rationale |
|---|---|---|
| Preheat temperature | 100–150°C | Reduce cooling rate, prevent HIC |
| Interpass temperature | 150–250°C | Control HAZ microstructure |
| Heat input | 1.5–4.0 kJ/mm | Balance penetration and HAZ width |
| Filler metal | E71T-8 / E80T-8 | Match base metal strength |
| Post-weld treatment | 200–250°C for 2h | Stress relief, reduce residual stress |
The welding sequence should be planned to minimize angular distortion and warping of the tube sections. Symmetrical welding patterns and balanced weld sequences are essential for maintaining the concentricity of the inner and outer tubes, which directly affects the effectiveness of the concrete confinement.
Engineering Practice Implications
The findings of this study have direct implications for the design and construction of composite columns in seismic regions. The enhanced ductility of the sandwich configuration provides superior energy dissipation capacity, which is crucial for earthquake-resistant design. However, engineers must carefully consider the constructability challenges, including the difficulty of concrete placement within the confined space between the two tubes, the need for specialized vibration equipment, and the potential for honeycombing if the concrete mix is not properly designed for workability.
The research also highlights the importance of quality control during fabrication. Non-destructive testing (NDT) of all welded joints, including ultrasonic testing (UT) for volumetric defects and magnetic particle testing (MT) for surface discontinuities, is essential to ensure the structural integrity of the composite system. Hydrostatic pressure testing of the inner tube prior to concrete placement can verify the watertightness of the welds and prevent concrete leakage during the casting process.
Key Questions and Reflections
Several questions remain open for future investigation. The long-term durability of the sandwich interface under environmental exposure, particularly in corrosive environments, requires accelerated aging studies. The seismic performance of the sandwich columns under multi-directional loading remains to be validated through shake table tests. Furthermore, the cost-benefit analysis of sandwich columns compared to conventional composite columns should be conducted to guide practical adoption in engineering projects.
In my experience with composite structure design, the key challenge lies not in the theoretical analysis but in ensuring that the design assumptions are validated through rigorous construction quality control. The sandwich configuration demands higher precision in fabrication and assembly, and any deviation from the design geometry can significantly affect the structural performance. Engineers should adopt a holistic approach that integrates materials selection, welding procedures, concrete technology, and quality assurance into a unified framework.
This research contributes valuable experimental data to the growing body of knowledge on multi-layer steel-concrete composite columns. The demonstrated improvements in load capacity and ductility make the sandwich configuration a promising solution for demanding structural applications, provided that the associated manufacturing and quality control challenges are adequately addressed through appropriate engineering practices and standardized procedures.