🤙🏼Earthquake Engineering Unit 8 – Performance-Based Earthquake Design

Key Concepts and Principles

• Performance-based earthquake design (PBED) focuses on ensuring structures meet specific performance objectives under seismic loading
• Involves defining performance levels (operational, immediate occupancy, life safety, collapse prevention) and associated earthquake hazard levels
• Considers the nonlinear behavior of structures during earthquakes and their ability to dissipate energy through ductile deformation
  • Ductility allows structures to undergo large deformations without collapse, enhancing life safety
• Utilizes probabilistic seismic hazard analysis (PSHA) to quantify the likelihood of different earthquake intensities at a given site
• Employs nonlinear static (pushover) and dynamic (time-history) analysis methods to assess structural performance
  • Pushover analysis provides insight into the overall force-displacement response and identifies potential weak links
  • Time-history analysis captures the dynamic response under specific ground motion records
• Incorporates performance-based design principles into the design process, such as capacity design and displacement-based design
• Emphasizes the importance of detailing for ductility, confinement, and energy dissipation (seismic detailing)

Historical Context and Evolution

• Early seismic design focused on elastic design methods and prescriptive code requirements (prior to 1970s)
• Shift towards performance-based approaches began in the 1990s, driven by the need for more resilient structures
• Major earthquakes (Loma Prieta 1989, Northridge 1994, Kobe 1995) highlighted limitations of traditional design methods
  • Significant damage and economic losses despite structures meeting code requirements
• Development of FEMA 273/274 guidelines (1997) and FEMA 356 (2000) established framework for PBED
• Advancement of computational tools and analysis techniques enabled more accurate prediction of structural performance
• Introduction of ASCE 41 (2006) and subsequent updates provided standardized procedures for seismic evaluation and retrofit
• Continued evolution and refinement of PBED methodologies through research and lessons learned from earthquakes (Chile 2010, New Zealand 2011, Nepal 2015)

Performance Objectives and Criteria

• Performance objectives define the desired level of structural performance under specific earthquake hazard levels
• Commonly used performance levels include operational, immediate occupancy, life safety, and collapse prevention
  • Operational: minimal damage, structure remains fully functional
  • Immediate occupancy: minor damage, structure safe for immediate occupancy after earthquake
  • Life safety: significant damage, but low likelihood of casualties
  • Collapse prevention: severe damage, but structure remains standing to allow safe evacuation
• Performance objectives are typically defined by stakeholders (owners, occupants, regulators) based on the structure's importance and intended use
• Quantitative performance criteria are established to assess whether performance objectives are met
  • Examples include maximum interstory drift ratios, residual drifts, and component damage limits
• Probabilistic approaches are used to account for uncertainties in seismic hazard, structural properties, and modeling assumptions
• Risk-based decision-making frameworks, such as FEMA P-58, provide a systematic approach to assess performance and optimize design decisions

Seismic Hazard Analysis

• Seismic hazard analysis quantifies the probability of exceeding various ground motion intensity measures at a given site
• Probabilistic seismic hazard analysis (PSHA) is the most widely used approach
  • Considers the contributions of all potential seismic sources and their associated uncertainties
  • Incorporates ground motion prediction equations (GMPEs) to estimate ground motion intensity as a function of magnitude, distance, and site conditions
• Seismic hazard curves represent the annual probability of exceedance for different ground motion intensity measures (peak ground acceleration, spectral acceleration)
• Uniform hazard spectra (UHS) provide spectral acceleration values at different periods for a given probability of exceedance
• Deaggregation of seismic hazard identifies the relative contributions of different magnitude-distance scenarios to the overall hazard
• Site-specific seismic hazard analysis may be required for critical structures or sites with complex geologic conditions
• Time-dependent seismic hazard analysis incorporates the temporal variation of earthquake occurrence rates and can be used for post-earthquake assessment and retrofit planning

Structural Modeling and Analysis Techniques

• Structural modeling involves representing the geometry, material properties, and boundary conditions of the structure
• Lumped plasticity models (concentrated hinges) are commonly used for nonlinear analysis of frame structures
  • Nonlinear behavior is concentrated at discrete hinge locations (beams, columns)
  • Requires calibration of hinge properties based on experimental data or detailed finite element analysis
• Distributed plasticity models (fiber sections) provide a more detailed representation of nonlinear behavior along the element length
  • Cross-sections are discretized into fibers with assigned material properties
  • Captures the spread of plasticity and interaction between axial force and bending moment
• Nonlinear static (pushover) analysis involves applying a monotonically increasing lateral load pattern to the structure
  • Provides insight into the overall force-displacement response and identifies potential failure mechanisms
  • Capacity curves represent the relationship between base shear and roof displacement
• Nonlinear dynamic (time-history) analysis subjects the structure to specific ground motion records
  • Captures the dynamic response and accounts for the effects of ground motion duration and frequency content
  • Multiple ground motion records are typically used to account for variability in seismic demand
• Incremental dynamic analysis (IDA) involves subjecting the structure to a suite of ground motions scaled to increasing intensity levels
  • Provides a comprehensive assessment of structural performance across a range of hazard levels
  • Fragility curves can be derived from IDA results to quantify the probability of exceeding different damage states

Design Strategies and Methods

• Capacity design principles ensure that yielding occurs in desired locations (beams) and prevents brittle failure modes (shear failure, column hinging)
  • Strong column-weak beam design promotes ductile behavior and prevents story collapse mechanisms
• Displacement-based design focuses on achieving target displacement profiles and limiting damage to acceptable levels
  • Stiffness and strength are selected to satisfy displacement criteria rather than force-based limits
• Energy dissipation devices (viscous dampers, friction dampers, yielding devices) can be incorporated to reduce seismic demands on the primary structure
  • Supplemental damping reduces the amplitude of vibration and limits the ductility demand on structural components
• Seismic isolation (lead-rubber bearings, friction pendulum bearings) decouples the structure from the ground motion and reduces the transmitted forces
  • Shifts the fundamental period of the structure and limits the seismic energy input
• Performance-based seismic retrofit strategies aim to upgrade existing structures to meet current performance objectives
  • Techniques include adding shear walls, braces, or dampers; strengthening columns and foundations; and improving detailing for ductility
• Resilient design strategies consider the post-earthquake functionality and recovery time of structures
  • Designing for reparability, redundancy, and robustness enhances the ability to quickly restore occupancy and functionality after an earthquake

Code Requirements and Standards

• Building codes and standards provide minimum requirements for seismic design to ensure life safety and collapse prevention
• International Building Code (IBC) and ASCE 7 are widely adopted in the United States
  • Specify seismic design criteria, load combinations, and detailing requirements for different seismic design categories
• ASCE 41 provides a standardized methodology for seismic evaluation and retrofit of existing buildings
  • Defines performance objectives, analysis procedures, and acceptance criteria for different structural systems and components
• FEMA P-58 offers a probabilistic framework for performance-based seismic design and risk assessment
  • Quantifies the probable consequences of earthquakes in terms of repair costs, downtime, and casualties
• Eurocode 8 is the European standard for seismic design of structures
  • Provides principles, requirements, and rules for the design of buildings and civil engineering works in seismic regions
• New Zealand Standard (NZS) 1170.5 and NZS 3101 provide seismic design provisions and concrete design requirements, respectively
  • Reflect the country's high seismicity and emphasis on ductile design and detailing
• Codes and standards are continuously updated based on research findings, field observations, and stakeholder feedback
  • Regular revisions incorporate advancements in seismic design methodologies and lessons learned from past earthquakes

Case Studies and Real-World Applications

• Seismic retrofit of the San Francisco City Hall (1998) demonstrated the effectiveness of performance-based design in preserving historic structures
  • Retrofit measures included base isolation, reinforcement of walls and diaphragms, and strengthening of foundations
• Christchurch Women's Hospital (2005) in New Zealand was designed using displacement-based design principles
  • Performed well during the 2010-2011 Canterbury earthquake sequence, remaining operational and serving as a post-disaster facility
• Retrofit of the Wallace F. Bennett Federal Building (2005) in Salt Lake City, Utah, utilized viscous dampers to reduce seismic demands
  • Dampers were installed in diagonal braces to dissipate energy and limit interstory drifts
• Seismic retrofit of the Oregon State Capitol (2008) employed a combination of base isolation and supplemental damping
  • Lead-rubber bearings and fluid viscous dampers were used to reduce seismic forces and improve performance
• Torre Mayor (2003) in Mexico City, one of the tallest buildings in seismic zones, incorporated a hybrid structural system
  • Dual system of steel moment frames and reinforced concrete shear walls provided lateral resistance and ductility
• Retrofit of the historic Anderton Court Shops (2016) in Seattle, Washington, used fiber-reinforced polymer (FRP) wrapping to strengthen unreinforced masonry walls
  • FRP provided confinement and increased the shear capacity of the walls without altering the building's appearance
• Seismic retrofit of schools in Istanbul, Turkey (2005-2015), aimed to reduce the vulnerability of educational facilities
  • Retrofit measures included adding shear walls, strengthening columns and foundations, and improving connections
• Post-earthquake reconstruction in Christchurch, New Zealand, following the 2010-2011 Canterbury earthquake sequence
  • Rebuilding efforts focused on implementing resilient design strategies and performance-based approaches to enhance the city's seismic resilience