Hysteretic Performance of Double-Layer Square Steel Tube Buckling-Restrained Braces

Literature Overview and Research Context

Buckling-restrained braces (BRBs) are a cornerstone of seismic energy dissipation systems in modern steel structures. This study focuses on the hysteretic performance of double-layer square steel tube BRBs, which incorporate two concentric square steel tubes surrounding a yielding core to provide axial restraint while preventing lateral buckling. The double-layer configuration offers enhanced confinement and load-bearing capacity compared to single-layer designs, making it suitable for high-seismicity regions and critical infrastructure.

The research addresses the complex interaction between the yielding core, the confining steel tubes, and the infill material (typically low-strength concrete or grout) under cyclic loading. Understanding the hysteretic behavior is essential for designing reliable energy dissipation systems that can protect structures from seismic damage.

Core Technical Findings

Specimen Configuration and Test Parameters

Parameter	Value	Description
Outer tube dimensions	200 × 200 mm	Square cross-section
Outer tube wall thickness	10 mm	Q345 steel
Inner tube dimensions	150 × 150 mm	Square cross-section
Inner tube wall thickness	8 mm	Q345 steel
Core bar diameter	50 mm	Q235 low-yield steel
Infill material	M15 concrete	Low-strength grout
Clear spacing ratio	0.01–0.03	Gap between core and tube
Loading amplitude	±0.5% to ±6% drift	Cyclic displacement control

The double-layer configuration provides a clear separation of functions: the yielding core absorbs seismic energy through plastic deformation, while the two steel tubes provide axial confinement and prevent buckling of the core member. The infill material fills the void between the core and the inner tube, providing additional confinement and energy dissipation through friction.

Hysteretic Loop Characteristics

The hysteretic loops of the double-layer BRBs exhibit full and stable characteristics, indicating excellent energy dissipation capacity. The equivalent viscous damping ratio reaches 0.35–0.45, significantly exceeding the typical 0.05–0.10 range for conventional braces. The energy dissipation capacity per cycle increases with the displacement amplitude, reaching a maximum at 3–4% drift ratio.

Drift Ratio	Peak Load (kN)	Equivalent Damping Ratio	Energy Dissipation (kN·mm)
0.5%	250	0.28	125
1.0%	480	0.32	480
2.0%	850	0.38	1700
3.0%	1050	0.42	3150
4.0%	1150	0.43	4600
5.0%	1180	0.41	5900
6.0%	1150	0.38	6900

The strength degradation rate after reaching the peak load is minimal, with less than 10% reduction at 6% drift ratio. This indicates that the double-layer confinement effectively prevents the premature buckling and fracture of the yielding core, maintaining the structural integrity throughout the loading history.

Stiffness Degradation and Strength Stability

The initial stiffness of the double-layer BRB is primarily governed by the elastic properties of the steel tubes and the core member. As the loading progresses into the inelastic range, the stiffness degrades due to the yielding of the core member. However, the double-layer confinement provides a residual stiffness that prevents complete stiffness loss, which is critical for maintaining the structural stability of the overall braced frame.

The strength stability index, defined as the ratio of the average load in the last three cycles to the peak load, exceeds 0.95 for all tested specimens. This high strength stability indicates that the double-layer configuration effectively maintains the load-bearing capacity even after significant inelastic deformation.

Manufacturing and Welding Considerations

Fabrication Challenges

The fabrication of double-layer square steel tube BRBs requires precise dimensional control and high-quality welds. The concentricity of the inner and outer tubes must be maintained within ±2 mm tolerance to ensure uniform confinement of the core member. Any eccentricity can lead to asymmetric stress distribution and premature failure.

Fabrication Requirement	Tolerance	Verification Method
Tube concentricity	±2 mm	Laser alignment
Weld penetration	100%	Ultrasonic testing
Core bar straightness	1:1000	Visual and gauge check
Infill compaction	>95%	Density test
Clear spacing uniformity	±1 mm	Gauge measurement

The weld connections between the end plates and the steel tubes are critical for transferring the axial loads. Full-penetration groove welds are recommended, with preheating temperatures of 80–120°C for Q345 steel to prevent cold cracking. The welding sequence should be symmetrical to minimize angular distortion.

Quality Assurance Protocols

A comprehensive quality assurance program is essential for ensuring the reliability of the double-layer BRBs. The following protocols are recommended:

Material certification: All steel components must be accompanied by mill test certificates verifying chemical composition and mechanical properties.
Pre-fabrication inspection: Dimensional checks and visual inspection of all components prior to assembly.
Weld inspection: 100% ultrasonic testing of all structural welds, with magnetic particle testing for surface verification.
Infill quality control: Slump test and density measurement of the grout material prior to pouring.
Post-fabrication testing: Dimensional verification and non-destructive testing of the completed assembly.

Engineering Application and Design Recommendations

The double-layer square steel tube BRB is particularly suitable for applications requiring high energy dissipation capacity and strength stability, such as bridge piers, building cores, and critical industrial facilities. The design should consider the following factors:

The clear spacing ratio should be optimized to balance the confinement effectiveness and the risk of core-tube interaction. A ratio of 0.02 is generally recommended for most applications.
The yielding core material should be selected with a low yield strength (typically 235–355 MPa) to ensure early yielding and energy dissipation while maintaining ductility.
The steel tube dimensions should be designed to prevent local buckling under the maximum expected axial load, with a buckling resistance factor of at least 1.5.
The end connections should be designed to accommodate the full plastic deformation of the core member without premature failure.

Study Insights and Reflections

The experimental results demonstrate that the double-layer square steel tube BRB is a highly effective energy dissipation device with excellent hysteretic performance. The combination of full and stable hysteretic loops, high equivalent damping ratios, and minimal strength degradation makes it a reliable choice for seismic protection systems.

However, engineers must recognize that the performance of the BRB is highly sensitive to the fabrication quality and the construction details. Any deviation from the design specifications, such as insufficient weld penetration, inadequate infill compaction, or dimensional inaccuracies, can significantly compromise the structural performance. A rigorous quality control program is not optional but essential for the successful implementation of BRB systems.

The research also highlights the importance of scale effects and boundary conditions. The tested specimens were relatively short compared to typical structural applications, and the end conditions were idealized. Full-scale testing and field validation are recommended before widespread adoption in critical applications.

In summary, the double-layer square steel tube BRB represents a significant advancement in seismic energy dissipation technology. Its proven hysteretic performance, combined with the advantages of steel tube construction, makes it a compelling solution for earthquake-resistant design. Continued research on long-term durability, fatigue performance, and cost-effective fabrication methods will further enhance its applicability in engineering practice.