🌉Bridge Engineering Unit 13 – Seismic Design & Performance of Bridges

Key Concepts in Seismic Design

• Seismic design aims to ensure bridges can withstand earthquake loads and maintain structural integrity
• Considers factors such as seismic hazard, site conditions, bridge configuration, and materials
• Utilizes performance-based design approach focuses on achieving specific performance objectives under different seismic hazard levels
• Incorporates ductility and energy dissipation mechanisms to prevent sudden collapse and allow for controlled damage
• Emphasizes the importance of redundancy and continuity in the load path to redistribute forces during seismic events
• Accounts for soil-structure interaction effects can significantly influence the seismic response of bridges
  • Includes kinematic interaction modifies the ground motion input to the bridge
  • Also includes inertial interaction alters the dynamic properties and response of the bridge
• Recognizes the need for regular maintenance and retrofit of existing bridges to enhance their seismic performance

Seismic Hazard Analysis

• Seismic hazard analysis assesses the probability and intensity of earthquake ground motions at a bridge site
• Involves identification and characterization of seismic sources such as faults and seismogenic zones
• Considers historical seismicity and geological evidence to estimate the likelihood and magnitude of future earthquakes
• Utilizes probabilistic seismic hazard analysis (PSHA) to quantify the uncertainty in ground motion predictions
  • PSHA combines the contributions from all potential seismic sources and ground motion models
  • Produces hazard curves and uniform hazard spectra for various return periods (e.g., 475 years, 2475 years)
• Deterministic seismic hazard analysis (DSHA) is also used to estimate the worst-case scenario ground motions from specific seismic sources
• Site-specific ground motion studies may be required for critical bridges or sites with complex geology
• Seismic hazard maps and design codes provide regional hazard information for preliminary design and assessment

Bridge Components and Their Seismic Behavior

• Bridge components include foundations, piers, abutments, bearings, and superstructure elements (girders, deck)
• Foundations transfer loads from the bridge to the ground and provide stability during earthquakes
  • Pile foundations are commonly used in seismic regions to resist lateral loads and prevent soil liquefaction
  • Spread footings may be used for bridges on competent soils with low seismic hazard
• Piers and columns are critical elements that support the superstructure and resist seismic forces
  • Ductile detailing and confinement reinforcement improve the deformation capacity and prevent brittle failure
  • Plastic hinges are designed to form at the base of piers to dissipate energy and limit damage to the superstructure
• Abutments provide vertical and lateral support to the superstructure and retain the approach embankment
  • Seat-type abutments allow for relative movement and reduce seismic forces on the superstructure
  • Integral abutments provide a rigid connection and transfer seismic forces to the foundation
• Bearings accommodate thermal movements and transmit vertical loads between the superstructure and substructure
  • Elastomeric bearings allow for limited horizontal movement and provide some energy dissipation
  • Seismic isolation bearings (lead-rubber, friction pendulum) decouple the superstructure from the substructure and reduce seismic forces
• Superstructure elements (girders, deck) distribute loads and provide a continuous load path
  • Continuous superstructures enhance redundancy and allow for redistribution of forces during seismic events
  • Expansion joints and hinges are vulnerable components that require special detailing to prevent unseating and collapse

Design Philosophies and Performance Objectives

• Seismic design philosophies have evolved from elastic design to ductile design and performance-based design
• Elastic design aims to keep the structure in the elastic range under seismic loads, but can result in uneconomical designs
• Ductile design allows for controlled inelastic behavior and energy dissipation, reducing seismic forces but accepting some damage
• Performance-based design sets specific performance objectives for different seismic hazard levels
  • Performance objectives are defined in terms of structural damage, functionality, and repair time
  • Common performance levels include fully operational, operational, life safety, and near collapse
• Design codes and standards (AASHTO, Caltrans) provide guidance on seismic design criteria and performance objectives
• Importance factor is used to adjust seismic design forces based on the bridge's importance and consequence of failure
• Seismic design category (SDC) is assigned based on the seismic hazard and site conditions, influencing the level of analysis and detailing required
• Service level and ultimate level earthquakes are considered to ensure serviceability and prevent collapse, respectively

Seismic Analysis Methods

• Seismic analysis methods predict the response of bridges under earthquake loads and guide the design process
• Equivalent static analysis (ESA) is a simplified method that represents seismic forces as static lateral loads
  • Suitable for regular bridges with short periods and low seismic hazard
  • Seismic forces are distributed along the bridge based on mass and stiffness
• Response spectrum analysis (RSA) is a linear dynamic analysis method that uses the structure's modal properties
  • Combines the peak responses of each mode using methods like SRSS (square root of sum of squares) or CQC (complete quadratic combination)
  • Accounts for higher mode effects and provides more accurate force and displacement estimates than ESA
• Time history analysis (THA) is the most sophisticated and computationally intensive method
  • Requires input of ground motion time histories compatible with the site's seismic hazard
  • Captures the nonlinear behavior of bridge components and soil-structure interaction effects
  • Used for critical bridges, irregular configurations, or when significant nonlinear response is expected
• Pushover analysis is a nonlinear static analysis method that assesses the bridge's capacity and identifies potential failure mechanisms
  • Applies incrementally increasing lateral loads to the bridge until a target displacement or collapse is reached
  • Provides insight into the bridge's ductility, strength hierarchy, and potential plastic hinge locations
• Seismic isolation and energy dissipation devices may require specialized analysis techniques to capture their behavior
• Analysis results are used to verify performance objectives, design components, and detail connections

Seismic Design Strategies and Techniques

• Seismic design strategies aim to control damage, prevent collapse, and ensure post-earthquake functionality
• Capacity design principle ensures a desirable hierarchy of strength and ductility in bridge components
  • Ductile components (plastic hinges in piers) are designed to yield and dissipate energy
  • Brittle components (foundations, superstructure) are designed to remain elastic and resist the maximum forces from ductile components
• Ductile detailing improves the deformation capacity and energy dissipation of bridge components
  • Closely spaced transverse reinforcement (hoops, spirals) provides confinement and prevents buckling of longitudinal reinforcement
  • Plastic hinge regions require enhanced detailing to accommodate large inelastic rotations without significant strength degradation
• Seismic isolation decouples the superstructure from the substructure and reduces seismic forces transmitted to the substructure
  • Lead-rubber bearings (LRBs) combine the flexibility of rubber layers with the energy dissipation of a lead core
  • Friction pendulum bearings (FPBs) use curved sliding surfaces to provide isolation and restore the superstructure to its original position
• Supplemental energy dissipation devices (dampers) can be installed to absorb seismic energy and reduce structural response
  • Viscous dampers utilize the resistance of fluid flow to dissipate energy
  • Hysteretic dampers (buckling-restrained braces, yielding metal dampers) dissipate energy through inelastic deformation of metals
• Restrainer cables and shear keys prevent unseating and provide transverse stability at expansion joints and hinges
• Seismic retrofit techniques are used to improve the performance of existing bridges
  • Jacketing of columns with steel or fiber-reinforced polymer (FRP) increases confinement and ductility
  • Adding isolation bearings or dampers reduces seismic forces and improves the response
  • Strengthening foundations and increasing seat widths enhance stability and prevent unseating

Performance Evaluation and Testing

• Performance evaluation assesses the seismic behavior and vulnerability of existing bridges
• Visual inspection and condition assessment identify structural deficiencies, damage, and deterioration
• Material testing (concrete cores, reinforcement samples) provides information on the strength and quality of bridge components
• Nondestructive testing techniques (ultrasonic, ground-penetrating radar) help evaluate the condition without causing damage
• Structural analysis using as-built plans and current seismic codes determines the bridge's capacity and identifies potential weaknesses
• Fragility analysis quantifies the probability of exceeding specific damage states under various seismic hazard levels
  • Fragility curves relate the ground motion intensity to the probability of damage
  • Used to prioritize bridges for retrofit and estimate post-earthquake functionality
• Experimental testing validates analytical models and provides insights into the seismic behavior of bridge components
  • Quasi-static cyclic testing applies reversing lateral loads to assess the hysteretic behavior and ductility of components
  • Shake table testing subjects scaled bridge models to simulated earthquake ground motions
  • Hybrid simulation combines physical testing of critical components with numerical modeling of the remaining structure
• Instrumentation and monitoring of bridges during earthquakes provide valuable data for model calibration and performance evaluation
• Post-earthquake reconnaissance and damage assessment inform future design practices and retrofit strategies

Case Studies and Lessons Learned

• Case studies of bridges that have experienced significant earthquakes offer valuable lessons for seismic design and performance
• San Francisco-Oakland Bay Bridge (Loma Prieta earthquake, 1989)
  • Failure of bolted connections and unseating of spans highlighted the need for improved detailing and restraint systems
  • Retrofit measures included strengthening connections, adding restrainer cables, and replacing vulnerable spans
• Hanshin Expressway (Kobe earthquake, 1995)
  • Collapse of steel box girder spans due to insufficient lateral restraint and poor weld quality
  • Emphasized the importance of ductile detailing, quality control, and consideration of soil-structure interaction
• Bolu Viaduct (Duzce earthquake, 1999)
  • Failure of tall, slender piers due to inadequate confinement and shear reinforcement
  • Highlighted the need for ductile detailing and capacity design principles in seismic regions
• Chile's highway bridges (Maule earthquake, 2010)
  • Good performance attributed to the use of seismic isolation, ductile detailing, and conservative design codes
  • Demonstrated the effectiveness of modern seismic design practices in reducing damage and maintaining functionality
• Lessons learned from case studies have influenced the evolution of seismic design codes and practices
  • Improved detailing requirements for ductility and confinement
  • Increased focus on preventing unseating and enhancing system redundancy
  • Adoption of performance-based design and consideration of multiple performance objectives
  • Recognition of the importance of regular maintenance and timely retrofit of existing bridges
• Sharing knowledge and experiences from past earthquakes promotes the continuous improvement of seismic design and resilience of bridges.