Zhuojin Pipe Fitting Co., LtdMechanical Performance of Hot-Bent Steel Tube CFST-RC Composite Joints
Literature Overview
The paper by Li Zhenbao et al., published in Journal of Harbin Institute of Technology (Vol. 54, No. 10, 2022, pp. 40-48), investigates the mechanical behavior of composite joints formed by hot-bent circular steel tubes inserted into reinforced concrete columns with external steel tube confinement (CFST-RC composite joints). Funded by the National Natural Science Foundation of China (51978014), this research is motivated by the design of a large-span truss structure in a sports arena where the chord tubes are bent and vertically inserted into underground RC frame columns at the arch foot.
Core Technical Content
Joint Configuration and Test Setup
Three composite joint specimens were designed and tested under combined loading conditions (chord compression and web tension). The key variables investigated include:
| Variable | Levels | Purpose |
|---|---|---|
| Hot-bent tube residual stress | Present/Absent | Evaluate residual stress effects |
| Bending angle | Multiple angles | Study angle-dependent behavior |
| Concrete infill in chord tube | Filled/Unfilled | Assess concrete contribution |
The hot-bending process introduces significant plastic deformation on the outer fiber of the bend, creating a residual stress distribution that fundamentally affects the joint's initial stiffness and load-bearing characteristics.
Hot-Bending Residual Stress Analysis
The hot-bending process involves heating the steel tube to a temperature typically in the range of 600-800°C, bending it to the required angle, and then cooling. This process creates:
- Outer fiber: Compressive residual stress (material was in tension during bending)
- Inner fiber: Tensile residual stress (material was in compression during bending)
- Neutral axis: Minimal residual stress
This residual stress pattern reduces the effective yield load of the tube cross-section at the bend root, which is precisely where the joint connection occurs. The implications for welding are significant, as the pre-existing residual stresses interact with welding-induced stresses, potentially leading to unexpected crack initiation sites.
Key Findings
- Failure mode: The dominant failure mode is local buckling of the chord tube at the root section. The joint bearing capacity is governed by the compressive capacity of the chord tube root.
- Residual stress effects: Hot-bending residual stresses reduce joint stiffness, with larger bending angles causing greater stiffness reduction. However, the bearing capacity is essentially unaffected by residual stresses, indicating that the ultimate strength is governed by material properties and geometry rather than initial stress state.
- Concrete infill contribution: Filling the steel tube with concrete significantly improves both joint stiffness and bearing capacity. This is consistent with the well-known confinement effect of concrete within steel tubes, where the concrete resists inward buckling of the tube wall.
- Size effect: Both joint stiffness and bearing capacity increase with steel tube diameter, following expected geometric scaling laws.
Welding and Fabrication Considerations
From a welding engineering perspective, this research raises several important practical issues:
Residual Stress Interaction
The combination of hot-bending residual stresses and welding residual stresses at the joint connection can lead to complex stress states. The welding process adds thermal cycling to an already stressed component, potentially causing:
- Local plastic deformation at the weld toe
- Stress redistribution that may exceed material yield limits
- Distortion that affects geometric fit-up
Recommended Welding Practices
For hot-bent tube joints, the following practices should be considered:
| Practice | Purpose |
|---|---|
| Pre-weld stress relief | Reduce hot-bending residual stresses before welding |
| Low heat input welding | Minimize additional thermal distortion |
| Backing ring with full penetration | Ensure complete fusion at critical root |
| Post-weld inspection | Detect cracks at stress concentration points |
| Controlled cool-down rate | Prevent martensitic transformation in HAZ |
Engineering Practice Integration
The finding that concrete infill significantly improves joint performance has direct implications for design optimization. In applications where weight is a concern, unfilled tubes may be acceptable if the stiffness requirements are not critical. However, for seismic applications where energy dissipation and ductility are paramount, concrete infill is strongly recommended.
The distinction between stiffness reduction and bearing capacity preservation is practically important. Residual stresses affect serviceability (deflection, vibration) but not ultimate limit state performance. This means that hot-bent tube joints can be safely used in structures where ultimate strength governs, provided that serviceability criteria are appropriately adjusted.
Key Reflections
This research addresses a practical structural engineering challenge with rigorous experimental and numerical methods. The connection between the hot-bending process and joint performance highlights the importance of considering manufacturing-induced effects in structural design—a principle that extends well beyond this specific application.
For pipeline and fitting engineers, the residual stress analysis methodology is directly applicable to understanding how pipe bending operations affect subsequent welding quality. The same principles of residual stress interaction apply to bent pipe spools that require field welding, where the bending residual stresses must be considered in the welding procedure specification.