🤙🏼Earthquake Engineering Unit 11 – Seismic Isolation and Energy Dissipation

Basics of Seismic Isolation

• Seismic isolation decouples a structure from the ground motion during an earthquake, reducing the seismic forces transmitted to the structure
• Involves installing flexible bearings or isolators at the base of the structure (lead-rubber bearings, friction pendulum bearings)
• Increases the natural period of the structure, shifting it away from the dominant periods of the earthquake ground motion
• Reduces the lateral stiffness of the structure, allowing it to move independently from the ground
• Limits the transfer of seismic energy into the structure, minimizing damage to structural and non-structural components
• Suitable for both new construction and retrofitting of existing structures
• Requires careful design and analysis to ensure proper performance and stability
• Commonly used in critical facilities (hospitals, emergency response centers) and high-value structures (museums, historic buildings)

Principles of Energy Dissipation

• Energy dissipation systems absorb and dissipate seismic energy, reducing the demand on the primary structural elements
• Convert kinetic energy from the earthquake into heat or other forms of energy through various mechanisms (friction, yielding, viscous damping)
• Supplement the inherent damping of the structure, which is typically low (2-5% of critical damping)
• Can be passive (requiring no external power) or active (requiring external power and control systems)
• Passive systems include:
  • Metallic yield dampers (ADAS, TADAS)
  • Friction dampers (Pall friction dampers, slotted bolted connections)
  • Viscoelastic dampers (solid or fluid-based)
• Active systems include:
  • Active mass dampers (AMD)
  • Active tendon systems
  • Semi-active dampers (magnetorheological dampers, variable orifice dampers)
• Improve the overall seismic performance of the structure by reducing drift, accelerations, and damage to non-structural components

Types of Seismic Isolation Systems

• Elastomeric bearings consist of alternating layers of rubber and steel plates, providing flexibility and damping
  • Low damping rubber bearings (LDRB) have lower damping (2-4%) and are often used in combination with supplemental damping devices
  • High damping rubber bearings (HDRB) have higher damping (10-20%) due to the addition of fillers or compounds to the rubber
  • Lead-rubber bearings (LRB) incorporate a central lead core that yields under seismic loads, providing additional damping and energy dissipation
• Sliding bearings allow the structure to slide relative to the foundation during an earthquake, limiting the transfer of seismic forces
  • Friction pendulum bearings (FPB) consist of a concave sliding surface and an articulated slider, providing a restoring force due to the curvature of the surface
  • Flat sliding bearings rely on the friction between the sliding surfaces to dissipate energy and limit the transfer of seismic forces
• Rolling bearings use rolling elements (balls, cylinders) to provide isolation and allow movement in multiple directions
• Hybrid systems combine different types of bearings or isolation devices to optimize performance and address specific design requirements
• Selection of the appropriate isolation system depends on factors such as the structure's weight, seismicity, site conditions, and performance objectives

Energy Dissipation Devices

• Metallic yield dampers dissipate energy through the inelastic deformation of metals (steel, lead, copper)
  • Added damping and stiffness (ADAS) devices consist of X-shaped steel plates that yield in flexure
  • Triangular added damping and stiffness (TADAS) devices have triangular-shaped plates that yield in flexure
  • Buckling-restrained braces (BRB) consist of a steel core encased in a steel tube filled with concrete, allowing the core to yield in tension and compression
• Friction dampers dissipate energy through the friction between sliding surfaces
  • Pall friction dampers use a series of steel plates with slotted holes, allowing relative movement and friction under seismic loads
  • Slotted bolted connections (SBC) have oversized holes in the connection plates, allowing sliding and friction between the connected elements
• Viscoelastic dampers use materials that exhibit both elastic and viscous behavior, providing damping and stiffness
  • Solid viscoelastic dampers consist of layers of viscoelastic material bonded between steel plates
  • Fluid viscoelastic dampers use a viscoelastic fluid (silicone, polyisobutylene) in a piston-cylinder assembly
• Tuned mass dampers (TMD) consist of a mass, spring, and damper attached to the structure to counteract its motion and reduce vibrations
• Supplemental damping devices are often used in combination with seismic isolation to further enhance the seismic performance of the structure

Design Considerations

• Seismic hazard analysis to determine the expected ground motion parameters (peak ground acceleration, response spectra) at the site
• Selection of the appropriate seismic isolation system based on the structure's characteristics, performance objectives, and site conditions
• Determination of the isolator properties (stiffness, damping, displacement capacity) to achieve the desired performance
• Consideration of the vertical load capacity and stability of the isolation system under gravity and seismic loads
• Analysis of the isolated structure using linear or nonlinear methods to evaluate its response and verify the design
  • Equivalent lateral force (ELF) method for preliminary design and simple structures
  • Response spectrum analysis (RSA) for more accurate estimation of the seismic demands
  • Time history analysis (THA) for complex structures or when nonlinear behavior is expected
• Design of the isolation interface, including the connections between the isolators and the structure, and the gap around the structure to accommodate the expected displacements
• Consideration of the effects of the isolation system on the structure's functionality, such as the need for flexible utility connections and access for maintenance
• Incorporation of supplemental damping devices, if necessary, to further reduce the seismic demands and improve the overall performance
• Verification of the design through peer review, testing, and commissioning to ensure the isolation system performs as intended

Performance Analysis

• Evaluation of the seismic performance of the isolated structure using various metrics and criteria
  • Peak floor accelerations to assess the safety and functionality of non-structural components and contents
  • Inter-story drifts to evaluate the potential for structural damage and the need for repairs
  • Residual displacements to determine the post-earthquake functionality and the need for re-centering
• Comparison of the performance of the isolated structure with that of a conventional fixed-base structure to quantify the benefits of seismic isolation
• Consideration of the uncertainties in the seismic hazard, isolator properties, and structural modeling through sensitivity analyses and probabilistic approaches
• Assessment of the performance of the isolation system under different earthquake scenarios, including near-fault ground motions and long-duration subduction zone events
• Evaluation of the effectiveness of supplemental damping devices in reducing the seismic demands and improving the overall performance
• Verification of the performance through instrumentation and monitoring of the isolated structure during and after earthquakes
• Updating of the performance analysis based on the observed behavior and new information, such as changes in the seismic hazard or the structure's conditions

Case Studies and Applications

• Seismic isolation of historic buildings to preserve their cultural value and structural integrity (San Francisco City Hall, USA; National Museum of Western Art, Japan)
• Retrofit of existing bridges using seismic isolation to improve their seismic performance and extend their service life (Golden Gate Bridge, USA; Rion-Antirion Bridge, Greece)
• Application of seismic isolation in hospitals and emergency response facilities to ensure their functionality and safety during and after earthquakes (USC University Hospital, USA; Olive View-UCLA Medical Center, USA)
• Use of seismic isolation in nuclear power plants to protect critical components and prevent the release of radioactive materials (Cruas Nuclear Power Plant, France; Koeberg Nuclear Power Station, South Africa)
• Implementation of seismic isolation in high-rise buildings to reduce the seismic demands and minimize damage to structural and non-structural components (Sendai AERU, Japan; Sabiha Gökçen International Airport, Turkey)
• Seismic isolation of industrial facilities and equipment to ensure business continuity and minimize economic losses (Fab 38 Intel Semiconductor Fabrication Plant, Israel; LNG tanks, various locations)
• Application of seismic isolation in residential buildings to protect occupants and minimize damage (Mustafa Kemal Atatürk's Mansion, Turkey; Casa Paraisópolis, Brazil)

Codes and Standards

• ASCE/SEI 7-16: Minimum Design Loads and Associated Criteria for Buildings and Other Structures
  • Chapter 17: Seismic Design Requirements for Seismically Isolated Structures
  • Provides design requirements, analysis procedures, and testing and inspection criteria for seismically isolated structures
• ASCE/SEI 41-17: Seismic Evaluation and Retrofit of Existing Buildings
  • Chapter 14: Seismic Isolation and Energy Dissipation
  • Provides guidelines for the seismic evaluation and retrofit of existing buildings using seismic isolation and energy dissipation devices
• FEMA P-1050: NEHRP Recommended Seismic Provisions for New Buildings and Other Structures
  • Chapter 12: Seismic Design Requirements for Seismically Isolated Structures
  • Provides recommended seismic design provisions for new seismically isolated structures, including design criteria, analysis procedures, and testing requirements
• EN 15129:2018: Anti-seismic devices
  • European standard specifying the requirements for the design, manufacturing, and testing of anti-seismic devices, including seismic isolation systems and energy dissipation devices
• ISO 22762:2018: Elastomeric seismic-protection isolators
  • International standard providing requirements and test methods for elastomeric seismic isolators, including rubber bearings and lead-rubber bearings
• Building-specific codes and standards, such as the International Building Code (IBC) and the California Building Code (CBC), which may have additional requirements for seismically isolated structures
• Local and regional seismic design guidelines and recommendations, which may provide further guidance on the application of seismic isolation and energy dissipation devices in specific areas or for particular types of structures