Zhuojin Pipe Fitting Co., LtdPure Bending Performance of Square Hollow Sandwich Steel Tube Concrete Members
Literature Overview
This study by Tao Zhong, Han Linhai, Zheng Yongqian, and Huang Hong from Fuzhou University investigates the pure bending behavior of square hollow sandwich steel tube concrete (CFST) members. The research was funded by the Fujian Provincial Science and Technology Program Major Project (2002H007) and published in Industrial Construction (Volume 34, Issue 1, 2004, pp. 6-9). Five pure bending specimens were tested with varying hollow ratios as the primary variable. The authors established stress-strain models for both steel and concrete, conducted numerical analysis, and provided simplified formulas for bending capacity and stiffness.
Core Technical Content
The sandwich steel tube concrete concept introduces a hollow cavity within the conventional CFST section, effectively creating a composite structure with an outer steel tube, an inner concrete layer, a hollow core, and potentially an inner steel tube. This configuration aims to reduce self-weight while maintaining structural integrity. The study systematically varied the hollow ratio to examine its influence on flexural behavior.
From a steel pipe manufacturing perspective, the key material interfaces and fabrication considerations include:
| Parameter | Typical Range | Manufacturing Implication |
|---|---|---|
| Outer steel tube wall thickness | 6-12 mm | Requires precision rolling or HFW welding |
| Inner steel tube (if present) | 4-8 mm | Must maintain concentricity with outer tube |
| Hollow ratio | 20%-60% | Affects concrete placement and compaction |
| Concrete strength grade | C30-C60 | High-strength concrete demands proper vibratory compaction through confined spaces |
| Steel grade | Q235-Q345 | Yield strength directly influences plastic hinge formation |
Welding and Fabrication Considerations
The fabrication of sandwich steel tube concrete members presents unique challenges from a welding and pipe processing standpoint. The connection between inner and outer tubes must be designed carefully. If welded, the following considerations apply:
- Welding process selection: GTAW (Tungsten Inert Gas Welding) is preferred for thin-walled inner tubes due to precise heat input control, while SMAW or FCAW may be used for thicker outer tubes.
- Heat-affected zone (HAZ) management: The confined space between inner and outer tubes creates restricted cooling conditions, which can lead to coarse-grained HAZ structures in the outer tube if heat input is excessive.
- Residual stress: The concentric tube geometry creates complex residual stress patterns. The inner tube weld creates circumferential residual stresses that interact with the outer tube's inherent hoop stresses from manufacturing.
- Geometric tolerances: The concentricity between inner and outer tubes must be maintained within ±2 mm to ensure uniform concrete layer thickness, which directly affects the composite action and confinement effectiveness.
Stress-Strain Model and Numerical Analysis
The authors developed constitutive models that account for the confinement effect of the steel tube on the concrete and the interaction between the two concrete layers (if a sandwich configuration exists). The key insight is that the hollow core reduces the effective concrete volume contributing to flexural resistance, but simultaneously reduces the compressive force on the compression side, potentially shifting the neutral axis and altering the plastic hinge mechanism.
The numerical results showed good agreement with experimental data, validating the proposed models. The simplified formulas provided for bending capacity and stiffness are practical for preliminary design but should be verified against detailed finite element analysis for critical applications.
Engineering Practice Implications
For pipe manufacturers and welding shops involved in producing steel tubes for such applications, the following quality control measures are essential:
- Dimensional accuracy: The outer diameter and wall thickness of the steel tube must comply with GB/T 8162 or GB/T 8163 tolerances to ensure proper fit within the sandwich assembly.
- Surface quality: Internal surface roughness affects the bond between steel and concrete. A surface roughness of 0.5-1.5 mm (achieved by mechanical or chemical treatment) is recommended for optimal composite action.
- Heat treatment: If the tube undergoes welding for fabrication (e.g., forming square sections from round tubes), post-weld heat treatment (PWHT) at 550-650°C for low-carbon steels is critical to relieve residual stresses and prevent delayed cracking.
- Non-destructive testing: UT (ultrasonic testing) for wall thickness verification, MT (magnetic particle testing) for surface defects, and RT (radiographic testing) for internal weld quality are all relevant to ensuring the tube's structural integrity in service.
Key Reflections
The sandwich concept represents an innovative approach to optimizing structural weight while maintaining capacity. From a manufacturing standpoint, the additional fabrication steps (inner tube installation, spacing, welding) increase production complexity and cost. The economic viability depends on whether the weight savings in transportation and erection outweigh the additional fabrication costs. The hollow ratio optimization identified in this study provides a useful guideline, but engineers must also consider the practical constraints of concrete placement in confined geometries.
This work contributes to the understanding of composite behavior in non-conventional CFST sections and offers a foundation for future research on optimized section design. The simplified formulas, while approximate, provide a practical tool for engineers during the preliminary design phase, reducing the need for extensive numerical modeling at early stages.