13.1 Seismic hazard analysis and ground motion characteristics

Seismic hazard analysis is crucial for bridge design in earthquake-prone areas. It helps engineers understand the likelihood and intensity of ground shaking a bridge might face. This knowledge shapes design decisions, ensuring bridges can withstand potential earthquakes.

Ground motion characteristics play a key role in how bridges respond to earthquakes. By studying factors like shaking intensity, frequency content, and duration, engineers can better predict how different bridge designs will perform during seismic events.

Seismic Hazard Analysis for Bridges

Probabilistic and Deterministic Approaches

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• Seismic hazard analysis quantifies likelihood of earthquake ground motions exceeding specified levels at a location over time
• Two primary methods used in seismic hazard assessment for bridge design
  • Deterministic Seismic Hazard Analysis (DSHA)
  • Probabilistic Seismic Hazard Analysis (PSHA)
• PSHA incorporates uncertainties in earthquake size, location, and recurrence through seismic source models and ground motion prediction equations (GMPEs)
• Return period relates to average time between occurrences of ground motions of specified intensity (50-year return period)

Hazard Curves and Disaggregation

• Seismic hazard curves represent annual probability of exceedance for different ground motion intensity measures (peak ground acceleration, spectral acceleration)
• Disaggregation of seismic hazard results identifies dominant earthquake scenarios contributing to hazard at a site
  • Magnitude-distance pairs
  • Helps in selecting appropriate ground motion records for analysis
• Time-dependent seismic hazard models adjust probabilities of future events based on elapsed time since last major earthquake on a fault
  • Accounts for stress accumulation on faults over time

Ground Motion Characteristics for Bridges

Intensity Measures and Spectral Representation

• Primary intensity measures quantify ground motion severity
  • Peak ground acceleration (PGA)
  • Peak ground velocity (PGV)
  • Peak ground displacement (PGD)
• Response spectra provide comprehensive representation of ground motion characteristics across structural periods
  • Acceleration response spectra most commonly used in bridge design
  • Velocity and displacement spectra also informative for certain applications
• Frequency content of ground motions affects dynamic response of bridges with different natural frequencies
  • Characterized by Fourier spectra or power spectral density functions
  • Influences resonance potential and energy distribution

Duration and Spatial Variability

• Duration of strong ground motion influences cumulative damage potential to bridge structures
  • Quantified by parameters like significant duration or bracketed duration
  • Longer durations can lead to increased cyclic degradation and fatigue effects
• Spatial variability of ground motions along bridge length can induce differential movements and additional seismic forces
  • Particularly important for long-span bridges
  • Can result in out-of-phase motions at different supports

Near-Fault Effects and Vertical Motions

• Near-fault effects can produce pulse-like ground motions with high velocity content
  • Directivity effects due to rupture propagation towards site
  • Fling-step from permanent ground displacement
  • Can lead to increased seismic demands on bridges (large displacement pulses)
• Vertical ground motions often significant in near-fault regions
  • Affect bridge components such as bearings and expansion joints
  • Can induce axial forces in columns and modify shear demands

Seismic Hazard Maps for Bridge Design

National Hazard Maps and Design Ground Motions

• Seismic hazard maps provide spatial representation of ground motion parameters for specified return periods or probabilities of exceedance
  • Parameters include PGA, spectral acceleration (SA) at different periods
• National seismic hazard maps (USGS) form basis for seismic design provisions in bridge design codes and standards
• Design ground motion in bridge engineering typically corresponds to specific probability of exceedance
  • 7% in 75 years for AASHTO LRFD Bridge Design Specifications
  • Translates to approximately 1000-year return period

Seismic Design Categories and Site-Specific Analyses

• Seismic design categories or zones defined based on level of seismic hazard
  • Influence required analysis methods and detailing provisions for bridges
  • Higher categories require more stringent design and detailing requirements
• Site-specific seismic hazard analyses necessary for important bridges or those in complex geological settings
  • Develop site-specific response spectra
  • Account for local fault systems and soil conditions

Spectral Representations for Design

• Uniform hazard spectra (UHS) derived from PSHA represent spectral accelerations with equal probability of exceedance across all periods
  • Often used as starting point for developing design spectra
  • Can be conservative for scenario-based assessments
• Conditional mean spectra (CMS) or conditional spectra (CS) provide more realistic representation of spectral shape for scenario-based assessments
  • Conditioned on spectral acceleration at a specific period
  • Better captures correlations between spectral ordinates

Site Effects on Ground Motion

Local Site Conditions and Classification

• Local site conditions can significantly amplify or de-amplify ground motions through site response effects
  • Characterized by soil type and shear wave velocity profiles
• Site classification systems categorize sites based on average shear wave velocity in upper 30 meters (Vs30)
  • NEHRP site classes (A through F)
  • Influence spectral amplification factors in design codes

Site Response Analysis and Topographic Effects

• One-dimensional equivalent linear and nonlinear site response analyses assess influence of local soil conditions on ground motion characteristics
  • Propagate bedrock motions through soil layers
  • Account for soil nonlinearity and damping
• Topographic effects modify ground motion characteristics
  • Basin effects can amplify and prolong shaking
  • Ridge amplification increases motion intensity at topographic highs
• Soil liquefaction potential must be assessed for bridge sites with saturated, loose granular soils
  • Can lead to large ground deformations and loss of soil strength

Site-Specific Factors and Soil-Structure Interaction

• Site-specific amplification factors modify rock ground motions to account for local soil conditions
  • Used when detailed site response analyses not performed
  • Often provided in design codes based on site class and spectral period
• Soil-structure interaction (SSI) alters dynamic characteristics of bridge system
  • Modifies effective ground motions experienced by structure
  • Can lead to period elongation and changes in damping
  • Important for short-period structures on soft soils