🤙🏼Earthquake Engineering Unit 9 – Seismic Evaluation and Retrofitting Structures

Key Concepts in Seismic Evaluation

• Seismic hazard analysis assesses the probability and severity of earthquakes in a specific region based on historical data, geological factors, and site-specific conditions
• Structural vulnerability refers to a building's susceptibility to damage or collapse during an earthquake due to factors such as age, design, materials, and construction quality
• Performance-based design aims to ensure that structures meet specific performance objectives under various levels of seismic intensity, balancing safety, functionality, and cost
• Nonlinear analysis techniques, such as pushover analysis and time-history analysis, help engineers understand the behavior of structures under seismic loads beyond the elastic range
• Seismic demand is the amount of force or displacement imposed on a structure during an earthquake, determined by factors such as ground motion intensity, soil conditions, and structural properties
• Capacity-demand ratio compares a structure's capacity to resist seismic forces with the expected seismic demand, indicating the likelihood of damage or collapse
• Fragility curves represent the probability of a structure exceeding a specific damage state (slight, moderate, extensive, or complete) as a function of seismic intensity measure (peak ground acceleration or spectral acceleration)

Structural Vulnerability Assessment

• Visual inspection is the first step in assessing a structure's vulnerability, focusing on identifying visible signs of damage, deterioration, or deficiencies that may affect seismic performance
• Material testing helps determine the strength and condition of structural components, such as concrete compressive strength, steel tensile strength, and reinforcement corrosion
• Nondestructive testing techniques, such as ultrasonic testing, ground-penetrating radar, and infrared thermography, provide valuable information about the internal condition of structural elements without causing damage
• Structural analysis using simplified or detailed models helps evaluate the expected seismic performance of a building, considering factors such as lateral load resisting system, irregularities, and potential failure mechanisms
  • Simplified analysis methods, such as the equivalent lateral force method, are suitable for regular, low-rise buildings and provide a conservative estimate of seismic forces
  • Detailed analysis methods, such as modal response spectrum analysis and nonlinear time-history analysis, are more accurate but computationally intensive and require specialized expertise
• Rapid visual screening is a quick, cost-effective method for identifying potentially vulnerable buildings in a large inventory, using a scoring system based on observable characteristics (construction type, irregularities, and soil conditions)
• Seismic risk prioritization helps allocate limited resources for retrofitting by ranking buildings based on their vulnerability, occupancy, and criticality (essential facilities, historic landmarks, or high-occupancy structures)

Seismic Performance Objectives

• Life safety performance objective aims to prevent collapse and minimize the risk of life-threatening injuries during a design-level earthquake, allowing safe evacuation of occupants
• Immediate occupancy performance objective ensures that a building remains functional and can be safely occupied immediately after an earthquake, with minimal damage and disruption to operations
• Collapse prevention performance objective focuses on preventing complete structural failure and loss of life during a rare, high-intensity earthquake, accepting significant damage and potential economic losses
• Operational performance objective requires a building to maintain its pre-earthquake functionality and appearance, with negligible damage and no interruption to essential services (hospitals, emergency response centers, and critical infrastructure)
• Stakeholder input is crucial in defining performance objectives, considering factors such as the building's purpose, occupancy, and the owner's risk tolerance and financial constraints
• Performance-based design allows for customized seismic performance objectives, tailoring the design and retrofitting strategies to meet specific needs and priorities
• Multi-level performance objectives combine different performance goals for various levels of seismic hazard (frequent, occasional, rare, or very rare events), providing a comprehensive approach to seismic risk management

Evaluation Methods and Tools

• ASCE 41 (Seismic Evaluation and Retrofit of Existing Buildings) provides a standardized methodology for assessing the seismic performance of existing buildings and designing retrofitting measures
• FEMA P-154 (Rapid Visual Screening of Buildings for Potential Seismic Hazards) offers a quick, non-technical method for identifying potentially vulnerable buildings based on observable characteristics and seismicity
• Nonlinear static analysis (pushover analysis) assesses the progressive yielding and failure of structural components under increasing lateral loads, providing insights into the building's capacity, ductility, and potential collapse mechanism
• Nonlinear dynamic analysis (time-history analysis) simulates the response of a structure to specific ground motion records, capturing the complex, time-dependent interaction between seismic forces and structural behavior
• Finite element analysis (FEA) allows for detailed modeling of structural components and connections, considering material nonlinearity, geometric nonlinearity, and local failure modes
• Experimental testing, such as shake table tests or quasi-static cyclic loading tests, validates analytical models and provides valuable data on the actual behavior of structural systems and components under seismic loads
• Seismic instrumentation and monitoring systems, including accelerometers and strain gauges, help assess the real-time performance of buildings during earthquakes and provide data for model calibration and post-event damage assessment

Common Retrofitting Techniques

• Addition of shear walls, typically made of reinforced concrete or steel, increases the lateral stiffness and strength of a building, reducing drift and improving overall seismic performance
• Steel bracing systems, such as concentric braced frames (CBFs) or eccentric braced frames (EBFs), provide additional lateral resistance and energy dissipation capacity, particularly in steel or composite structures
• Seismic isolation systems, such as elastomeric bearings or friction pendulum bearings, decouple the building from the ground motion, reducing the seismic forces transmitted to the structure and minimizing damage
• Damping devices, including viscous dampers, hysteretic dampers, and tuned mass dampers, absorb seismic energy and reduce the amplitude of structural vibrations, mitigating damage and improving occupant comfort
• Fiber-reinforced polymer (FRP) wrapping strengthens and enhances the ductility of concrete columns, beams, and walls, preventing brittle failure modes and increasing the structure's resilience
• Retrofit of foundations, such as enlarging footings, adding micro-piles, or improving soil conditions, ensures adequate load transfer and prevents foundation failure during earthquakes
• Non-structural component bracing and anchorage, including securing suspended ceilings, light fixtures, and mechanical equipment, minimizes the risk of falling hazards and ensures the functionality of essential systems

Case Studies and Real-World Applications

• Retrofit of the San Francisco City Hall, a historic landmark built in 1915, involved seismic isolation, damping devices, and structural strengthening, preserving its architectural integrity while improving seismic performance
• Seismic retrofit of schools in Istanbul, Turkey, as part of the Istanbul Seismic Risk Mitigation and Emergency Preparedness (ISMEP) project, aimed to reduce the vulnerability of over 1,500 school buildings and ensure the safety of students and staff
• Upgrade of the Golden Gate Bridge in San Francisco, California, included the installation of energy dissipation devices, strengthening of tower shear links, and retrofit of the suspension cable system to withstand a 8.3 magnitude earthquake
• Retrofit of the Los Angeles City Hall, a 32-story building constructed in 1928, involved the installation of 526 seismic isolators and 68 viscous dampers, making it one of the largest base-isolated buildings in the world
• Seismic strengthening of the Acropolis Museum in Athens, Greece, combined traditional and innovative techniques, such as titanium reinforcement and carbon fiber wrapping, to protect the museum's invaluable collections while respecting its architectural heritage
• Retrofit of the Husky Stadium at the University of Washington, Seattle, used a combination of seismic isolation, damping devices, and structural strengthening to mitigate the risk of liquefaction and ensure the safety of spectators
• Seismic upgrade of the Oregon State Capitol Building in Salem, Oregon, involved the installation of a base isolation system, reinforced concrete shear walls, and the strengthening of existing structural elements to improve its seismic performance

Codes and Standards for Retrofitting

• ASCE 41 (Seismic Evaluation and Retrofit of Existing Buildings) provides a comprehensive framework for assessing the seismic performance of existing buildings and designing retrofitting measures, considering various performance objectives and analysis methods
• FEMA 356 (Prestandard and Commentary for the Seismic Rehabilitation of Buildings) offers guidelines for the seismic rehabilitation of existing buildings, focusing on performance-based design and the use of innovative technologies
• International Building Code (IBC) includes provisions for the seismic design of new buildings and the evaluation and retrofit of existing structures, ensuring a consistent approach across jurisdictions
• Eurocode 8 (Design of Structures for Earthquake Resistance) provides a framework for the seismic design of new buildings and the assessment and retrofitting of existing structures in Europe, considering regional seismicity and construction practices
• New Zealand Society for Earthquake Engineering (NZSEE) Guidelines provide recommendations for the seismic assessment and retrofit of existing buildings in New Zealand, tailored to the country's unique seismic hazard and building stock characteristics
• Japan Building Disaster Prevention Association (JBDPA) Standard for Seismic Evaluation and Retrofit of Existing Reinforced Concrete Buildings offers guidance for the assessment and strengthening of existing RC buildings in Japan, reflecting the country's extensive experience with seismic retrofitting
• China Code for Seismic Design of Buildings (GB 50011) includes provisions for the seismic evaluation and retrofit of existing buildings, considering the country's diverse seismic hazard and construction practices

Future Trends in Seismic Retrofitting

• Performance-based earthquake engineering (PBEE) is an emerging approach that integrates seismic hazard analysis, structural modeling, damage assessment, and loss estimation to provide a more comprehensive understanding of seismic risk and inform decision-making
• Smart materials, such as shape memory alloys (SMAs) and magnetorheological (MR) dampers, offer adaptive and controllable properties that can enhance the seismic performance of structures and enable real-time response to earthquakes
• Resilient design strategies aim to minimize downtime and facilitate rapid recovery after an earthquake, considering factors such as redundancy, robustness, and the use of prefabricated or modular components for easy repair and replacement
• Sustainability considerations in seismic retrofitting, such as the use of recycled materials, energy-efficient technologies, and the minimization of embodied carbon, align with the goals of green building and contribute to the overall environmental performance of structures
• Integration of seismic retrofitting with other building upgrades, such as energy efficiency improvements, accessibility modifications, and fire safety enhancements, provides a holistic approach to building modernization and risk reduction
• Advancements in sensing and monitoring technologies, including wireless sensor networks, fiber optic sensors, and computer vision techniques, enable real-time structural health monitoring and early damage detection, facilitating timely maintenance and retrofit interventions
• Collaboration between engineers, architects, and stakeholders is essential for the successful implementation of seismic retrofitting projects, ensuring that technical solutions are compatible with aesthetic, functional, and socio-economic considerations