15-EN15129 Clause 6: Displacement Dependent Devices (DDD) – Standards and Typical Products

1. Scope and Definition

Clause 6 governs two main categories of DDD: linear devices (LD) and nonlinear devices (NLD). A key defining feature of DDD is that they do not bear vertical loads. Additionally, buckling-restrained braces (BRBs) that provide additional damping within structures are explicitly classified as DDD. Supplementary technical information about DDD is available in Annex D of EN15129.

These devices are exclusively intended for structures in seismic regions compliant with the EN 1998 series, with the primary goal of enhancing structural resilience by regulating dynamic behavior and dissipating seismic energy, thereby working synergistically with the overall seismic protection system.

2. Classification of DDD

Linear Devices (LD): Characterized by linear or quasi-linear mechanical behavior, LDs are used to optimize structural dynamic properties. Their minor nonlinearity and energy dissipation capabilities are designed to be compatible with linear structural modeling, ensuring simplicity and accuracy in engineering analysis.

Nonlinear Devices (NLD): Exhibiting strong nonlinear behavior, NLDs improve structural dynamic performance by introducing significant nonlinearity and/or energy dissipation. Due to their complex mechanical response, they must be fully incorporated into nonlinear structural modeling to ensure reliable seismic design.

3. Key Performance and Compliance Requirements

Clause 6 specifies rigorous performance criteria to ensure DDD reliability under seismic conditions:

Displacement and Load Resistance: DDD must withstand specified displacement or load limits (whichever is reached first), with a minimum safety factor (γ) of 1.1. For components integrated into isolation systems, these factors are adjusted to align with the displacement capacity of isolation devices (refer to Clause 8 of EN15129).

Force-Displacement Curve: The curve must not show a descending trend when displacement or load reaches the specified design limits, ensuring stable load-bearing capacity during seismic events.

Cyclic Stability: The effective stiffness and effective damping of DDD must remain stable across cycles. For cycles i ≥ 2, deviations from the 3rd cycle (a reference point for stable performance) must not exceed 10%.

Residual Displacement: Under serviceability limit state (SLS) seismic actions, the zero-force residual displacement must be minimized (recommended to be 5% of the design displacement or at least 10mm, whichever is larger), reducing post-earthquake structural damage and repair costs.

4. Material and Testing Requirements

Materials for DDD are categorized into "core materials" (critical for cyclic seismic performance) and "structural materials" (for load-bearing functions). Core materials such as elastomers, steel, and shape memory alloys (SMA) must meet strict European standards:

Elastomers: Low-damping and high-damping elastomers must comply with the requirements in Tables 10 and 11 of Clause 8, respectively, with verified bonding strength to substrates.

Steel: Must conform to EN 10025, EN 10083, or EN 10088 series standards, ensuring ductility and fatigue resistance.

Special Materials (e.g., SMA): Must meet existing European standards, with additional tests for phase transformation characteristics, cyclic performance, and temperature adaptability.

Testing is a cornerstone of Clause 6, including material type testing, factory production control (FPC) testing, device type testing, and pre-installation testing. Type testing is required when there are changes to device geometry, materials, or connection systems, while FPC testing (sampling rate ≥ 2%) ensures consistent performance in mass production.

DDD products are widely used in European and American seismic engineering, with distinct applications based on their linear or nonlinear characteristics. Below is a summary of mainstream products, their core features, and typical use cases:

1. Linear Devices (LD)

LDs are ideal for projects requiring linear structural modeling, offering stable stiffness adjustment with minimal nonlinear energy dissipation. Common types include:

Linear Metal Dampers

Core Features: Manufactured from carbon steel or low-alloy steel, these dampers exhibit near-ideal linear force-displacement behavior without significant yield stages. They rely on elastic deformation to adjust structural natural periods, with weak energy dissipation capabilities.

Applications: Suitable for small to medium-sized frame structures requiring dynamic property optimization with low additional energy dissipation needs, such as seismic retrofitting of existing industrial buildings.

Compliance Highlights: Materials must meet EN 10025 standards, with cyclic stability verified through type testing.

Linear Viscoelastic Dampers

Core Features: Utilizing low-damping elastomers (compliant with Table 10 of Clause 8), these dampers offer quasi-linear damping characteristics and stable effective stiffness. They combine stiffness adjustment with mild energy dissipation, compatible with linear dynamic modeling.

Applications: Ideal for curtain walls, equipment foundations, and auxiliary stiffness adjustment components in buildings located in moderate seismic zones with stable temperature conditions.

Compliance Highlights: Dynamic shear testing is required to verify performance, with material parameter deviations (due to supply, temperature, etc.) meeting the limits in Table 4 of Clause 6.

Load-Bearing Buckling-Restrained Braces (BRBs)

Core Features: Classified as DDD providing additional damping, these BRBs prioritize linear load-bearing and stiffness characteristics with weak energy dissipation. The core material is high-strength steel (EN 10083), and the sleeve prevents core buckling to ensure consistent tension and compression capacity.

Applications: Lateral force-resisting systems in high-rise steel frames and large-span spatial structures, where both load-bearing capacity and dynamic property optimization are required.

Compliance Highlights: Type testing must include connection systems, with residual displacement meeting SLS requirements.

2. Nonlinear Devices (NLD)

NLDs are critical for high-seismic-intensity regions, leveraging strong nonlinearity to dissipate significant seismic energy. They require nonlinear structural modeling and are available in various configurations:

Metal Yield Dampers

Core Features: Constructed from low-yield steel (e.g., LY100, LY160, LY225) with low yield strength and high ductility. The force-displacement curve shows distinct bilinear behavior, with stable post-yield stiffness and excellent cyclic energy dissipation.

Subtypes: Shear-type, bending-type, and axial-yielding dampers, adaptable to different installation spaces and force requirements.

Applications: New building seismic design and existing building retrofitting, particularly in high-seismic-intensity zones for frame and shear wall structures.

Compliance Highlights: Materials require monotonic tension and cyclic performance testing. After accelerated aging (14 days at 70°C), performance changes must not exceed 20%.

Friction Dampers

Core Features: Energy dissipation is achieved through relative sliding between contact surfaces, with a rectangular hysteretic force-displacement curve (strong nonlinearity). The friction coefficient is stable, and energy dissipation is directly related to displacement amplitude. They do not rely on material yield, ensuring long service life and minimal maintenance.

Subtypes: Plate-type, cylindrical-type, and spherical friction dampers, suitable for multi-directional displacement requirements.

Applications: Long-span bridges, large-span stadiums, high-rise structures, and other projects with large displacement demands. Ideal for scenarios requiring long-term stable energy dissipation and low maintenance, such as nuclear power plant auxiliary facilities.

Compliance Highlights: Long-term friction testing is required to verify wear stability. The ratio of upper to lower limits of material properties for metal components must not exceed 1.4.

Displacement-Amplified Dampers

Core Features: Integrating mechanical amplification mechanisms (e.g., toggle, scissor, gear), these dampers amplify small structural displacements (3–4 times) to enhance energy dissipation efficiency in small-deformation scenarios, exhibiting strong nonlinearity.

Working Principle: Toggle braces, scissor trusses, or gear-rack mechanisms amplify inter-story displacement, transmitting the amplified displacement to internal damping elements (e.g., low-yield steel cores, friction components) to achieve "small displacement, large energy dissipation."

Applications: Structures with small lateral deformation, such as shear wall structures, tube structures, and rigid industrial plants. Also suitable for projects requiring sufficient energy dissipation during minor earthquakes.

Compliance Highlights: Strength and stability of the amplification mechanism must be verified. Cyclic testing at 25%, 50%, and 100% of maximum displacement is required.

Shape Memory Alloy (SMA) Dampers

Core Features: Utilizing shape memory alloys (e.g., Ni-Ti alloys), these dampers dissipate energy and achieve self-centering through phase transformation (martensite to austenite). The force-displacement curve shows nonlinear hysteretic behavior with minimal residual displacement.

Working Principle: During earthquakes, SMA wires/bars undergo plastic deformation (martensitic transformation) to dissipate energy. Post-earthquake, the material automatically returns to its original shape through phase reversal, significantly reducing structural residual displacement.

Applications: Historic buildings (requiring minimal post-earthquake damage), precision equipment factories, and bridge expansion joints. Ideal for projects prioritizing both energy dissipation and self-centering capabilities.

Compliance Highlights: Testing for phase transformation characteristics (DSC), monotonic tensile failure, and cyclic performance is required, covering the service temperature range and strain rates. Materials must meet existing European standards.

Standard Alignment: Ensure DDD products comply with both EN15129 Clause 6 and local seismic standards (e.g., Eurocode 8 in Europe, ASCE 7 in the United States) for cross-border projects.

Modeling Compatibility: Select LDs for linear structural modeling and NLDs for nonlinear modeling to ensure accurate seismic response analysis.

Quality Assurance: Prioritize products with complete type testing certificates and strict FPC processes to guarantee performance consistency in mass production.

Application Specificity: Match DDD types to structural characteristics (e.g., displacement demand, stiffness) and environmental conditions (e.g., temperature, corrosion risk) to optimize seismic performance.