Zhuojin Pipe Fitting Co., LtdBuckling-Restrained Braces with Layered Steel Pipes and Long-Slot Holes
Literature Overview
The paper by Zhang Dongbin, Pan Peng, Wang Mengzi, Deng Kailai, and Chen Yabin (2016, China Civil Engineering Journal, Vol. 49, No. 12, pp. 9-15) presents an innovative design for buckling-restrained braces (BRBs) using layered steel pipes with long-slot holes. The authors designed and tested five specimens under axial low-cycle reverse loading to evaluate the influence of hole ratio and hole quantity on bearing capacity, deformation capacity, hysteretic energy dissipation, and failure modes.
Design Philosophy and Material Selection
The brace concept combines three different sizes of Q235 steel pipe profiles, with the core member subjected to axial force weakened by long-slot holes. This design philosophy is particularly interesting from a materials engineering standpoint because it deliberately exploits the ductile behavior of mild carbon steel (Q235, yield strength approximately 235 MPa) while controlling the buckling behavior through geometric weakening.
| Design Parameter | Range Tested | Effect on Performance |
|---|---|---|
| Hole ratio (area reduction) | Variable | Controls yielding pattern and deformation capacity |
| Number of long-slot holes | Variable | Affects stress concentration and energy dissipation |
| Core pipe material | Q235 | Ductile, low yield, suitable for controlled yielding |
| Outer restraint pipes | Q235 (three sizes) | Provide compressive restraint |
| Additional equivalent damping ratio | 30%–42% | High energy dissipation capability |
| Number of test specimens | 5 | Reverse cyclic axial loading |
Welding and Fabrication Considerations
The layered steel pipe construction requires precise welding between the core pipe and the outer restraint pipes. From a welding metallurgy perspective, the following points deserve emphasis:
- Weld joint design: The connection between core and outer pipes must accommodate large inelastic deformations without weld fracture. Fillet welds with adequate leg length and complete penetration butt welds are critical.
- Heat-affected zone (HAZ) concerns: Q235 steel has good weldability, but the HAZ in thick-walled sections can still experience microstructural changes. For multi-pass welding, interpass temperature control is essential.
- Slot hole fabrication: The long-slot holes in the core pipe introduce stress concentrations at hole edges. The hole edges must be deburred and free from machining burrs to prevent crack initiation under cyclic loading.
- Weld sequence: The welding sequence for the layered pipe assembly should be planned to minimize residual stress accumulation, as residual stresses can interact with cyclic stress to accelerate fatigue damage.
Hysteretic Performance Analysis
The test results demonstrate that the braces with reasonable hole ratios exhibit full hysteretic loops, indicating stable energy dissipation throughout the loading cycles. The additional equivalent damping ratio of 30%–42% is remarkably high, suggesting that the long-slot hole design effectively promotes yielding in the core member while the outer pipes prevent buckling.
Defect Analysis and Countermeasures
| Potential Defect | Location | Cause | Countermeasure |
|---|---|---|---|
| Lack of fusion | Core-outer pipe weld | Insufficient heat input | Increase welding current, improve fit-up |
| Undercut at slot edges | Long-slot hole boundaries | Excessive welding speed | Reduce travel speed, use backing strip |
| HAZ cracking | Welded joints | High residual stress + hydrogen | Preheat, post-weld heat treatment (PWHT) |
| Stress concentration | Hole corners | Sharp edge geometry | Fillet the hole edges, control hole dimensions |
Engineering Practice Integration
For seismic isolation and energy dissipation applications, this BRB design offers a practical alternative to traditional BRBs that rely on confined concrete or granular materials. The all-steel construction eliminates concerns about concrete degradation, moisture ingress, and corrosion of internal materials. However, the welding quality of the layered pipe assembly becomes the single point of failure, and rigorous quality control per GB/T 19866 or ISO 3834 is mandatory.
Study Insights and Outlook
The layered pipe BRB concept demonstrates that geometric weakening of ductile steel members can achieve excellent energy dissipation without complex internal confinement systems. The key engineering challenge lies in ensuring that weld joints between the layered pipes maintain integrity under large cyclic deformations. Future work should investigate the fatigue life of these braces under low-amplitude cyclic loading, as well as the effect of different steel grades (Q345, Q390) on the energy dissipation characteristics. The proposed design is particularly promising for bridge seismic retrofitting and nuclear facility isolation systems where high damping ratios are required.