Zhuojin Pipe Fitting Co., LtdHysteretic Performance of Double-Layer Square Steel Tube Buckling-Restrained Braces
Literature Overview
This study investigates the hysteretic behaviour of double-layer square steel tube buckling-restrained braces (BRBs) under cyclic loading conditions. BRBs are widely used in seismic design of steel structures to provide energy dissipation through controlled inelastic deformation of a slender core element. The double-layer configuration introduces an outer steel tube surrounding the conventional BRB, creating a nested structural system that enhances confinement, stability, and damage tolerance. This research provides valuable data on the energy dissipation capacity, degradation characteristics, and failure modes of this advanced brace configuration.
Structural Configuration and Design Parameters
The double-layer square steel tube BRB consists of three primary components: an inner slender core plate (the energy-dissipating element), a middle restraint jacket (concrete-filled or steel-restrained), and an outer square steel tube providing additional confinement and stability.
| Component | Material | Typical Dimensions | Function |
|---|---|---|---|
| Inner core plate | Q345/Q390 steel | Width: 100–200 mm, Thickness: 6–12 mm | Energy dissipation through plastic deformation |
| Middle restraint jacket | Concrete (C40–C60) + steel mesh | Gap: 15–30 mm from core plate | Prevent buckling of core plate |
| Outer square tube | Q345/Q420 steel | Side: 200–400 mm, Thickness: 8–16 mm | Additional confinement, stability |
| End connections | Welded end plates | Thickness: 20–40 mm | Force transfer to structural frame |
The core plate is typically designed with a yield length ratio (Ly/L) of 0.3–0.5, ensuring that plastic deformation is concentrated within the designated zone while maintaining elastic behaviour at the connections.
Hysteretic Performance Analysis
The cyclic loading tests reveal several important characteristics of the double-layer BRB:
| Performance Indicator | Single-Layer BRB | Double-Layer BRB | Improvement |
|---|---|---|---|
| Peak load capacity | 1.00 (reference) | 1.15–1.30 | 15–30% |
| Equivalent damping ratio | 12–18% | 18–25% | 50–60% |
| Cumulative energy dissipation (at 4% drift) | 1.00 (reference) | 1.40–1.80 | 40–80% |
| Load degradation at 6% drift | 15–25% | 8–15% | Reduced degradation |
| Number of stable cycles at 3% drift | 8–12 | 15–22 | 75–100% increase |
The double-layer configuration significantly improves the stability of the hysteretic loops. In single-layer BRBs, the concrete jacket may crack or spall under cyclic loading, reducing confinement effectiveness and leading to premature buckling of the core plate. The outer steel tube provides continuous confinement even after the concrete jacket has cracked, maintaining the restraint on the core plate throughout the entire loading history.
Failure Modes and Defect Analysis
The failure modes observed in the tests provide critical insights into design and fabrication requirements:
- Core plate buckling (most common): Occurs when the restraint gap is insufficient or the concrete jacket has deteriorated. The buckling wavelength is typically 1.5–3.0 times the core plate width.
- Concrete jacket cracking: Radial cracks develop at the mid-length of the jacket when the lateral expansion of the core plate exceeds the concrete's tensile capacity. In the double-layer system, these cracks are arrested by the outer tube.
- Outer tube local buckling: Can occur at the connection regions where stress concentrations develop. This is mitigated by providing stiffening rings at the end plates.
- Weld cracking at end connections: Fatigue cracking at the weld root between the core plate and end plate is a critical failure mode. The weld should be designed as a full-penetration groove weld with a weld throat thickness not less than 0.7 times the core plate thickness.
Welding Process and Quality Control
The fabrication of double-layer BRBs involves several critical welds that directly influence seismic performance:
- Core plate to end plate welds: Must be full-penetration groove welds using GTAW (TIG) or SAW processes. The heat input should be controlled between 1.0–1.8 kJ/mm to prevent excessive HAZ softening. Preheating to 100°C is recommended for steel thicknesses exceeding 10 mm.
- Outer tube to end plate welds: These welds carry the primary structural loads and must achieve full fusion. FCAW or SAW processes are suitable, with a minimum weld penetration of 95% of the tube wall thickness.
- Inner tube (if present) to outer tube welds: These welds maintain the concentricity of the double-layer configuration. TIG welding is preferred for the precision required, with a root gap of 2–3 mm and a single-pass or multi-pass technique depending on the gap size.
Quality inspection requirements include:
- Full UT examination (ISO 17635 Level 2) of all structural welds
- MT inspection of weld surfaces for surface-breaking defects
- Visual inspection (VT) of all welds per ISO 17637
- Hydrostatic or pneumatic pressure test of the concrete jacket to verify watertightness before concrete placement
Engineering Practice Applications
The double-layer BRB is particularly suitable for:
- Seismic retrofitting of existing steel structures where the outer tube can be installed around existing braces without structural disruption.
- High-seismicity regions (seismic intensity VIII–IX) where enhanced energy dissipation capacity is required.
- Long-span bridges and viaducts where the brace must withstand repeated seismic events without significant degradation.
- Structures with stringent serviceability requirements where the outer tube provides additional protection against accidental damage.
Key Questions and Reflections
The study raises an important question about the long-term durability of the double-layer configuration. The inner concrete jacket is susceptible to carbonation and chloride ingress, particularly in coastal or industrial environments. The outer steel tube provides physical protection, but the gap between the inner and outer tubes may accumulate moisture and corrosive agents. Proper drainage and ventilation provisions should be incorporated into the design.
Additionally, the construction sequence is critical. The concrete jacket must be placed after the inner core plate and restraint system are assembled, and the outer tube must be installed after the concrete has achieved sufficient strength (typically 70% of design strength). This sequential construction process increases project duration and requires careful coordination.
Study Insights and Outlook
The double-layer square steel tube BRB represents a significant advancement in seismic energy dissipation technology. The improved hysteretic performance, enhanced stability, and damage tolerance make it a promising solution for high-seismicity applications. Future research should address the performance of the brace under combined axial and bending loads, the effect of temperature on the hysteretic behaviour, and the development of simplified design equations that can be incorporated into standard seismic design codes. The technology also warrants investigation for application in offshore platforms and marine structures where cyclic loading from wave action presents similar challenges to seismic loading.