Unbonded Buckling-restrained Energy-dissipation Braces

The Buckling Restrained Energy-dissipation Brace (BRB), also known as "Buckling-Restrained Brace" or "Energy-Dissipation Brace", is called "Buckling Restrained Braces (BRB)" in Taiwan, China, and "Unbonded Buckling-restrained Braces (UBB)" in the United States and Japan. In mainland China, it is generally referred to as "Buckling Restrained Energy-dissipation Braces (BREB)" or "Buckling-Restrained Energy-Dissipation Braces(BRB)". It is an innovative seismic energy-dissipation product that cleverly integrates the dual functions of braces and energy-dissipation dampers. The core of the buckling-restrained brace is made of low-yield-point steel, which allows large plastic deformation under axial force to achieve energy dissipation. It plays a crucial role in seismic reinforcement and reconstruction projects of various new buildings and existing buildings, significantly enhancing the stability and seismic performance of building structures and safeguarding people's lives and property.

After the Wenchuan Earthquake, buckling-restrained braces have been widely promoted and applied due to their unique characteristics of safety, economy, and design flexibility.
The three major principles of seismic fortification for building structures are:
" Undestroyed in minor earthquake;
Retrofitted in moderate earthquake;
Uncollapsed in Massive earthquake.".
With the application of buckling-restrained braces, the seismic performance of building structures can be further improved to fully achieve.

★Minor earthquakes: Excellent economic performance
Due to the absence of compression stability issues, buckling-restrained braces have 2-10 times higher component bearing capacity than ordinary braces under wind load and minor earthquakes, with longer braces offering greater capacity improvements. Under the same bearing capacity, their cross-sections can be significantly reduced compared to ordinary braces, making the structural lateral stiffness more flexible and increasing the period. A longer structural period reduces seismic response, particularly the seismic acceleration. After adopting buckling-restrained braces, all natural periods increase, reducing the seismic response of each mode by generally 10-25%. If the structure is controlled by seismic conditions, the reduction in seismic action allows all component cross-sections to be reduced, typically lowering the overall construction cost by 10-30%.
★Moderate earthquakes: Remaining intact
Buckling-restrained braces have a clear yield bearing capacity, yielding first to dissipate energy under moderate earthquakes, acting as a "fuse" for the structure to protect important main components like beams and columns from yielding. Additionally, under general moderate earthquakes, the plastic deformation of buckling-restrained braces is not significant, and most can continue to be used after inspection.
★Major earthquakes: Retrofitting easily.
When working in the elastoplastic stage, buckling-restrained braces feature strong deformation capacity and excellent hysteretic performance, similar to high-performance energy-dissipation dampers, enhancing the structure's resistance to major earthquakes and ensuring safety. After major earthquakes, buckling-restrained braces with significant yield deformation can be easily replaced without affecting building use. In contrast, traditional beam-end plastic hinge energy-dissipation damage requires large-area temporary floor support or floor demolition during beam removal, severely impacting building use.
★Aftershocks: Being uncollapsed
With the increasing importance of buildings, some structures need not only to avoid collapse under major earthquakes but also to remain standing during aftershocks. By reasonably arranging buckling-restrained braces, the main structure is protected from excessive plastic deformation, ensuring vertical load-bearing components do not collapse during aftershocks and achieving the effect of "non-collapse during aftershocks".

Under external forces such as earthquakes, the axial force on the brace is entirely borne by the core material located at the center. The core material, made of specific steel, can quickly enter the yielding state under alternating axial tension and compression to efficiently dissipate seismic energy. Meanwhile, the external constraint mechanism, such as steel pipes or steel pipe concrete, provides strong lateral constraints to the core material, effectively preventing buckling during compression and ensuring stable energy dissipation. Due to the Poisson effect, the core material expands when compressed. Therefore, an unbonded material or a narrow air layer is intentionally set between the core material and the filler (such as mortar or formulated concrete) to significantly reduce or eliminate the force transmitted from the core material to the filler and outer casing during axial loading, ensuring the external constraint mechanism focuses on constraint functions without bearing axial loads.

Compared with steel moment-resisting frames and ordinary braced frames, the Buckling Restrained Energy-dissipation Frame (BREF) has the following characteristics:

1. Compared with steel moment-resisting frames, BREF has high linear elastic stiffness under minor earthquakes, easily meeting code deformation requirements.
2. Due to its ability to yield in tension and compression, BREF eliminates the buckling problem of traditional concentric braced frames, offering stronger and more stable energy dissipation capacity during strong earthquakes.
3. BRB is connected to gusset plates via bolts or hinges, avoiding on-site welding and inspection, making installation convenient and economical.
4. The brace component acts as a replaceable "fuse" in the structural system, protecting other components from damage and allowing easy replacement of damaged braces after major earthquakes.
5. With easily adjustable stiffness and strength, BREF enables flexible design. Additionally, its hysteretic curve can be conveniently simulated using bilinear hysteretic models in general finite element analysis software (e.g., SAP2000, ETABS, MIDAS).
6. In seismic retrofitting, BREF is more advantageous than traditional bracing systems, as capacity design can increase foundation costs for the latter.

(▲) Horizontal Composition

1. Core Unit
The core unit is the main load-bearing part of the buckling-restrained brace, typically made of steel, such as low-yield-point steel, ordinary steel, or special steel.
1) It has various cross-sectional forms, such as I-shape, cross-shape, and H-shape. Different cross-sections suit different engineering needs; for example, I-shaped sections are suitable for small-span structures, while H-shaped sections have high flexural stiffness for large-span structures.
2) The core unit yields and dissipates energy under axial force, absorbing seismic energy through repeated tension and compression deformations. Its design considers mechanical performance indicators such as yield strength, ultimate strength, and elongation to ensure effective energy dissipation during earthquakes.

2. Constraint Unit
The constraint unit restricts buckling of the core unit, maintaining stable mechanical properties under large deformations.
1) It is generally made of steel pipes, concrete, or other high-performance materials. Steel pipe constraint is a common form, with the pipe filled with concrete or other fillers to increase the unit's stiffness and stability.
2) A certain gap is usually left between the constraint unit and the core unit to allow free expansion and contraction of the core unit during deformation. The gap size is reasonably designed based on factors such as the core unit's dimensions, material properties, and engineering requirements.

3. Sliding Mechanism
The sliding mechanism is located between the core unit and the constraint unit to reduce friction, ensuring free sliding of the core unit during deformation. Its design considers factors such as friction force, durability, and installation convenience to maintain good performance of the buckling-restrained brace during long-term use.

4. Connection Nodes
Connection nodes are the interfaces between the buckling-restrained brace and the main structure, transmitting the brace's forces to the main structure.

4.1 Welded Connection
1), Advantages:
a) High connection strength: Welding ensures a very firm connection, capable of withstanding large tensile, compressive, and shear forces to ensure reliable connection.
b) Good integrity: Welded connections integrate the brace with the main structure, facilitating force transmission and dispersion and improving overall structural stability.
c) Relatively simple construction: Welding can be efficiently completed during factory prefabrication, especially for skilled welders.
2), Disadvantages:
a) High welding quality requirements: Welding quality is affected by factors such as welder skills, welding processes, and environmental conditions. Poor quality may lead to defects like cracks and pores, affecting strength and reliability.
b) Non-detachable: Once welded, connections are difficult to disassemble or replace, causing challenges for later maintenance or replacement.
c) Heat-affected zone issues: Welding generates heat-affected zones, potentially altering steel properties and reducing strength and toughness.
4.2 Bolted Connection
1), Advantages:
a) Good detachability: Bolted connections allow easy disassembly and replacement, facilitating post-installation maintenance.
b) High installation precision: Adjusting bolt tightening torque can precisely control connection stiffness and preload, ensuring reliability.
c) Low component damage: No high-temperature welding avoids thermal effects on steel, reducing performance degradation.
2), Disadvantages:
a) Relatively lower connection strength: Compared to welded connections, bolted connections have lower strength, particularly under large dynamic loads, where bolts may loosen or slip.
b) Larger space requirement: Bolted connections need installation space, which may be limited in compact structural areas.
c) Higher cost: Requires numerous bolts, nuts, washers, and other components, increasing costs.
4.3 Pin Connection
1), Advantages:
a) Good rotational performance: Pin connections allow a certain degree of rotation, adapting to structural deformation under earthquakes and reducing internal forces.
b) Easy installation: Simple installation without complex welding or bolt-tightening operations, enabling fast construction.
c) Low dimensional requirements: Suitable for different sizes of braces and main structures.
2), Disadvantages:
a) Limited load-bearing capacity: Primarily suitable for small tensile and shear forces; larger loads may require other connection methods.
b) Wear issues: Long-term use may cause wear between pins and hole walls, affecting reliability, requiring regular inspection and maintenance.
c) High design and machining precision requirements: Precise pin-hole matching is essential to ensure connection performance.

(▲▲) Longitudinal Composition

Vertically, the buckling-restrained energy-dissipation brace consists of a middle energy-dissipation segment and two end connection segments. The core material of the energy-dissipation segment is specially designed to yield first and dissipate energy during earthquakes. The connection segments, made of high-strength steel, are firmly connected to structural components (beams, columns, etc.) via welding, bolting, or pinning to ensure efficient load transmission.

1. Excellent Energy Dissipation Capacity:

As a displacement-dependent metal yielding damper, unbonded buckling-restrained energy-dissipation braces have excellent ductility and hysteretic energy-dissipation capabilities. Under minor earthquakes, they act as ordinary braces, providing strong lateral stiffness to resist wind and minor seismic effects. Under moderate to major earthquakes, they rapidly transform into high-efficiency energy-dissipation components, significantly reducing structural seismic response by dissipating large amounts of seismic energy.
2. High and Stable Bearing Capacity:

Due to their unique structure, these braces can yield in both tension and compression. Their axial bearing capacity depends solely on the core material's cross-sectional area and strength design value, independent of parameters like slenderness ratio, ensuring stable and reliable performance under various complex conditions.
3. Structural "Fuse" Function:

During severe earthquakes, buckling-restrained braces enter the yielding and energy-dissipation state before main structural components, acting like an electrical fuse to protect the main structure from severe damage at their own expense and significantly enhancing seismic safety.
4. Reduced Adjacent Component Forces:

By overcoming the inherent defect of ordinary braces compression buckling, these braces exhibit minimal difference in bearing capacity between compression and tension. This significantly reduces internal forces in adjacent components (including foundations), allowing for smaller component cross-sections and lowering overall structural costs.
5. Precisely Controllable Mechanical Properties:

They have clear and adjustable yield bearing capacity, stiffness, and strength. Using general finite element analysis software (e.g., SAP2000, ETABS, MIDAS), their hysteretic curves can be conveniently simulated using bilinear hysteretic models, providing strong support for structural design and analysis and enabling engineers to accurately grasp their mechanical behavior for scientific design.
6. Outstanding Durability:

With good aging and fatigue resistance, their mechanical properties remain stable over long-term use, requiring minimal maintenance or replacement and reducing lifecycle maintenance costs. Additionally, their simple structure and easy construction shorten the construction period and improve efficiency.

(▲) Classification

Common buckling-restrained energy-dissipation braces are mainly classified into two categories based on constraint methods:

1. Steel Sleeve + Mortar (or Concrete) Composite Constraint Type, Code C:

This type uses steel sleeves and internal mortar or concrete to provide strong constraints to the core material, widely applied in various building structures.
2. All-Steel Structure Constraint Type, Code S:

This type uses all-steel components for core material constraint, featuring a compact

structure and convenient installation, excelling in projects with high space requirements or harsh construction conditions.

Classification by Earthquake Intensity
3. High-Fatigue BRB: Suitable for high-intensity zones, with yield bearing capacity ≥4000kN and fire resistance rating of Grade II.
4. Two-Stage/Multi-Stage BRB: Adaptable to different earthquake magnitudes, with yield bearing capacity adjustable between 50%-150%.

(▲) Marking

The marking of buckling-restrained energy-dissipation braces consists of the product name "BRB", classification code, yield bearing capacity (unit: kN), and yield displacement (unit: mm). For example, a steel sleeve + mortar composite constraint brace with a yield bearing capacity of 2500kN and yield displacement of 1.5mm is marked as: BRB-C×2500×1.5. This clear marking system helps users quickly identify key product parameters during selection and use.

Our company's buckling-restrained energy-dissipation braces are designed, manufactured, and inspected in strict accordance with relevant national and industry standards to ensure excellent quality and reliable performance. Specific standards include:
1, China:

1) Code for Seismic Design of Buildings (GB 50011) and Technical Specification for Energy-Dissipation and Shock-Absorbing Structures (JGJ 297) specify design and application requirements for energy-dissipation braces.
2) Code for Seismic Design of Building Structures (GB50011-2010): Product performance tests and indicators strictly comply with requirements in Section 12.3, ensuring the braces play their intended role in structural seismic design and providing reliable seismic protection.
3) Building Energy-Dissipation Dampers (JG/T209-2012): Performance tests, indicators, and inspection standards adhere to detailed regulations in Sections 6.4, 7.4, 8, and 9. Every link, from raw material selection to production process control and final inspection, is strictly monitored to meet the highest industry standards.

2, International:

1) United States: Seismic Design Code (ASCE/SEI 7) and Seismic Design Code for Steel Structures (AISC 341). For unbonded braces (often called Buckling-Restrained Braces, BRB in the U.S.), AISC 341 specifies design, calculation methods, and construction requirements.
2) Japan: As an early adopter of unbonded brace research and application, Japan refers to them as Unbonded Buckling-restrained Braces (UBB) due to their structural characteristics and special constraint mechanisms. Relevant standards include the Code for Seismic Design of Building Structures, which, though lacking independent clauses for unbonded braces, addresses design principles, calculation methods, and construction requirements for structures using energy-dissipation components like unbonded braces in relevant seismic design provisions.
3) Eurocode 8 - Design of Structures for Earthquake Resistance: Proposes design methods for unbonded braced frames (BRBF) through extensions and improvements to Eurocode 8.

1. Production Flow

2. Processing Key Steps

1), Cutting Technology
a) Traditional Method: Flame cutting, with high temperature and large heat-affected zones, significantly impacts plate properties, produces abundant slag, often requiring rework, and may necessitate secondary machining for functional segments.
b) Current Method: Our company uses plasma cutting + laser cutting technology, which offers better inclination control and smaller heat-affected zones, minimal slag, and excellent fine-cutting effects, improving production efficiency and processing quality.
2), Unbonded Materials
Specific thicknesses of rubber-based rolled materials with self-adhesive surfaces are used.

1. Quality and Performance Requirements
1) Appearance: Surfaces should be flat, free of mechanical damage, rust, burrs, and clearly marked. Welded connections must meet Grade I weld standards.
2) Raw Materials: Core units preferably use low-yield-point steel. If other steels are used, they must comply with GB/T 700 or GB/T 3077, with elongation >25%, yield ratio <80%, and impact toughness >27J at room temperature.
3) Constraint Units: Typically made of carbon structural steel or alloy structural steel, with properties complying with GB/T 700 or GB/T 3077.
4) Mechanical Properties: Include yield bearing capacity, maximum bearing capacity, yield displacement, ultimate displacement, elastic stiffness, second stiffness, and hysteretic curve.
5) Durability: Requires fatigue resistance and corrosion resistance.

2. Testing Methods
1) The performance testing of raw material steel for buckling-restrained energy-dissipating braces shall be conducted in accordance with GB/T 228 and GB/T 7314.
2) Mechanical performance testing method: The test adopts a force-displacement hybrid control loading system. Before the specimen yields, force control with graded loading shall be used, and the loading increment shall be reduced appropriately before approaching the yield load. After yielding, displacement control shall be adopted, with each level of displacement loading amplitude taking multiples of the yield displacement as the increment, and each level of loading can be repeated three times.
3) For durability, the number of fatigue cycles shall be ≥30 times, using a fixed-displacement cyclic load test. The displacement shall be the design displacement corresponding to the location of the buckling-restrained brace, and the number of cycles when the maximum bearing capacity decreases by 15% shall be determined as the fatigue life. Corrosion resistance shall be observed visually, and routine anti-rust treatment shall be implemented.

3. Sampling Requirements
For the same project, the same type, and the same specification, 3% of the quantity shall be sampled. When the number of damper products of the same type and specification is small, 3% of the total quantity can be sampled from the same type of dampers, but not less than 2 pcs. The sampled products can be returned to the customer after non-destructive testing, but the tested products shall not be used in the main structure.

4. Finished Product Testing
1) Mechanical performance testing
2) Axial bearing capacity test: Test the bearing capacity of the buckling-restrained brace under axial compression and tension. The test shall be carried out in accordance with relevant standards, and data such as yield force, ultimate bearing capacity, and deformation of the brace shall be recorded.
3) Low-cycle repeated loading test: Simulate the working state of the buckling-restrained brace under seismic action. Important performance indicators such as the hysteresis curve and energy dissipation capacity of the brace can be obtained through the test.
4) Appearance quality inspection
5) Conduct a comprehensive inspection of the appearance of the finished buckling-restrained brace, including surface flatness, paint quality, and identification. Ensure that the brace has no obvious defects in appearance and clear and complete markings.

5. Testing Equipment & Testing reports

Testing of BRB in Industrial University of Peking.

6. Products Patent

(▼) Pre-installation Preparation

1. Technical Preparation
1) Familiarize with the design drawings and understand the requirements for the model, specification, quantity, installation location, and connection method of the buckling-restrained braces.
2) Prepare an installation construction plan, clarifying the construction process, technical key points, quality control measures, and safety precautions.
3) Conduct technical disclosure to construction personnel to ensure they master the installation technical requirements and operation methods.

2. Material Preparation
1) Inspect the product quality of the buckling-restrained braces, including appearance quality, dimensional deviations, and mechanical properties, to ensure compliance with design requirements and relevant standards.
2) Prepare installation materials such as connecting parts, bolts, nuts, and washers to ensure their quality and specifications meet requirements.

3. Site Preparation
1) Clean the installation site to ensure the structural surface at the installation location is flat, clean, and free of debris and oil stains.
2) Measure the structural dimensions of the installation location, determine the installation position and elevation of the buckling-restrained braces, and make marks.

(▼▼) Installation Process
1. Brace Positioning
1) Accurately place the buckling-restrained brace at the installation position according to the design drawings and site marks.
2) Use temporary supports or lifting tools to fix the buckling-restrained brace to prevent movement or tilting during installation.
2. Connection Node Installation

1) Welded connection: Perform welding at the connection part, and the welding process shall comply with relevant standards and specifications. After welding, inspect the weld quality to ensure compliance with requirements.
2) Bolted connection: Install connecting parts such as bolts, nuts, and washers at the connection part, and use wrenches to tighten the bolts to ensure firm connection. The tightening torque of the bolts shall meet design requirements.
3) Pin connection: Insert the pin into the hole of the connection part and install the pin fixing device to ensure firm pin connection. The installation accuracy of the pin shall meet design requirements.

3. Brace Adjustment
1) After installation, adjust the buckling-restrained brace to ensure its position, elevation, and perpendicularity meet design requirements.
2) Use tools such as jacks and chain blocks to fine-tune the buckling-restrained brace to ensure a tight and reliable connection with the main structure.

4. Anti-corrosion Treatment
Carrying out anti-corrosion treatment on the exposed parts of the buckling-restrained brace, such as painting anti-corrosion paint or galvanizing, to prevent corrosion during use.

(▼▼▼) Post-installation Inspection
1. Appearance Inspection
1) Inspect the appearance quality of the buckling-restrained brace, including whether there is damage, deformation, rust, etc.
2) Inspect the appearance quality of the connection nodes, including whether the welds are full, the bolts are tightened, and the pins are firmly installed.
2. Dimensional Inspection
1) Inspect the dimensional deviations of the buckling-restrained brace, including length, width, and height, to ensure compliance with design requirements.
2) Inspect the dimensional deviations of the connection nodes, including hole spacing, hole diameter, bolt spacing, etc., to ensure compliance with design requirements.
3. Other Inspections
Welding flaw detection, paint film thickness, etc.

The installation of buckling-restrained braces must be carried out strictly in accordance with design requirements and construction plans to ensure installation quality and safety. During installation, paying attention to construction safety, taking protective measures, and avoiding safety accidents.

(▼▼▼▼) Installation Site Photos

X. Application Scenarios

1. High-rise Buildings: In high-rise buildings, the impact of wind loads and seismic actions on the structure is particularly significant. Buckling-restrained energy-dissipating braces can provide strong lateral stiffness for high-rise buildings, effectively reducing the displacement response of the structure under wind and seismic loads, and ensuring the structural safety of high-rise buildings. At the same time, their excellent energy dissipation capacity can dissipate a large amount of seismic energy during strong earthquakes, protect the main structure from serious damage, and gain valuable time for personnel evacuation and rescue in high-rise buildings.
2. Large-Span Spatial Structures: For large-span spatial structures such as gymnasiums, convention centers, and airport terminals, due to their large spatial span and complex structural forms, the requirements for structural stability and seismic performance are extremely high. Buckling-restrained energy-dissipating braces can be flexibly arranged at key positions of large-span spatial structures to effectively improve the overall seismic performance of the structure through their own energy dissipation, ensuring that the large-span spatial structure remains stable and avoids serious accidents such as collapse during natural disasters such as earthquakes, thus protecting the safety of internal personnel and facilities.
3. Seismic Retrofitting of Old Buildings: For a large number of existing old buildings, their structural seismic performance often fails to meet the requirements of current seismic codes. Using buckling-restrained energy-dissipating braces for seismic retrofitting has the advantages of simple construction, little impact on the original structure, and remarkable retrofitting effects. By adding buckling-restrained energy-dissipating braces at appropriate positions in old buildings, the seismic capacity of the structure can be effectively improved, the service life of old buildings can be extended, and they can continue to safely serve people.
4. Key Defense Buildings such as Schools and Hospitals: Buildings with dense personnel and of great significance to social stability and public safety, such as schools and hospitals, have stricter requirements for seismic 设防. Buckling-restrained energy-dissipating braces, with their excellent seismic performance and reliable quality, can provide all-round seismic protection for these key defense buildings, ensuring that the building structure does not collapse during earthquakes, internal personnel can be protected in a timely and effective manner, and favorable conditions are created for subsequent rescue and recovery work.

XI. Company Strength and Services
Our company has an excellent professional R&D and design team, whose members all have rich experience in structural engineering and seismic design, and can provide personalized buckling-restrained energy-dissipating brace solutions according to different customer needs. At the same time, the company is equipped with advanced production equipment and a complete quality inspection system, strictly controlling the quality of each link from raw material procurement to product production to ensure that every product leaving the factory meets high-quality standards.
In terms of after-sales service, the company has established a perfect customer service network to provide customers with comprehensive technical support and after-sales services. Whether it is installation guidance, problem consultation during use, or after-sales maintenance, we will wholeheartedly provide customers with timely, efficient, and professional services with a quick response, so that customers have no worries.
"Professionalism makes buildings safer." We are committed to providing customers with the highest quality products and the most complete services, working together to create a safer and more reliable building structure system.

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