13.1 Seismic hazard analysis and ground motion characteristics
4 min read•july 30, 2024
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, , 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
- (DSHA)
- (PSHA)
- PSHA incorporates uncertainties in earthquake size, location, and recurrence through seismic source models and equations (GMPEs)
- 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 (, )
- Disaggregation of seismic hazard results identifies dominant earthquake scenarios contributing to hazard at a site
- Helps in selecting appropriate ground motion records for analysis
- 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)
- (PGV)
- (PGD)
- Response spectra provide comprehensive representation of ground motion characteristics across structural periods
- 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 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 or
- Longer durations can lead to increased cyclic degradation and fatigue effects
- 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
- 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
- (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
- Translates to approximately 1000-year return period
Seismic Design Categories and Site-Specific Analyses
- 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
- (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
- (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
- categorize sites based on average shear wave velocity in upper 30 meters (Vs30)
- (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
- 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
- (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
Key Terms to Review (32)
AASHTO LRFD Bridge Design Specifications: The AASHTO LRFD Bridge Design Specifications are guidelines published by the American Association of State Highway and Transportation Officials (AASHTO) for the design of highway bridges using the Load and Resistance Factor Design (LRFD) method. This method incorporates a reliability-based approach to ensure safety and performance under various loads, accounting for uncertainties in materials, loads, and environmental conditions. It provides a framework for engineers to analyze and design bridges that can withstand the stresses of traffic, including considerations for seismic activity and lessons learned from bridge failures.
Acceleration response spectra: Acceleration response spectra is a graphical representation that illustrates how structures respond to ground motion during seismic events, specifically showing the maximum acceleration experienced by a structure at various natural frequencies. This concept is crucial for understanding how different structures will behave during an earthquake, as it helps engineers evaluate the seismic performance of buildings and bridges based on their dynamic characteristics.
Bracketed Duration: Bracketed duration refers to the time interval during which ground motion is considered significant in the context of seismic hazard analysis. It helps define the effective period over which seismic effects are analyzed, focusing on the durations of strong ground motion that can lead to structural damage or failure. Understanding bracketed duration is crucial for evaluating the impact of earthquakes on structures and designing appropriate mitigation strategies.
Conditional Mean Spectra: Conditional mean spectra refer to the average spectral representation of ground motions conditioned on a specific seismic hazard level or event. This concept is crucial in seismic hazard analysis, as it helps engineers and researchers understand how ground motion characteristics vary depending on different levels of earthquake intensity, allowing for better assessment of structural responses and design criteria.
Design ground motions: Design ground motions refer to the simulated seismic waves that engineers use to assess how structures will respond to earthquakes. These motions are derived from seismic hazard analyses and reflect the characteristics of potential earthquake events at a specific location, helping to ensure that structures can withstand the expected levels of seismic activity.
Deterministic seismic hazard analysis: Deterministic seismic hazard analysis (DSHA) is a method used to evaluate the potential ground shaking at a specific location due to seismic sources, focusing on the maximum expected earthquake scenarios. This approach involves identifying and characterizing potential earthquake sources, estimating their magnitudes, and calculating the resultant ground motion, providing crucial input for designing structures to withstand seismic events. By utilizing precise parameters, DSHA plays a significant role in assessing the risk associated with earthquakes and informs the seismic design process.
Fourier spectra: Fourier spectra represent the frequency components of a signal, providing insights into how different frequencies contribute to the overall shape and behavior of that signal. In the context of seismic hazard analysis, Fourier spectra are essential for understanding ground motion characteristics, as they allow engineers and seismologists to analyze how seismic waves propagate through different geological conditions and how structures respond to these waves.
Frequency content: Frequency content refers to the range of frequencies present in a signal, such as ground motion during an earthquake. Understanding frequency content is essential for analyzing how seismic waves interact with structures, as different frequencies can lead to varying levels of stress and response in those structures. This concept helps engineers assess the impact of seismic forces and design buildings that can better withstand earthquakes.
Ground motion prediction: Ground motion prediction refers to the process of estimating the expected ground shaking intensity at a specific location due to an earthquake. This involves using statistical and computational models that take into account various factors such as seismic source characteristics, wave propagation, and site conditions. Accurate predictions are vital for assessing seismic hazards and designing structures that can withstand earthquake forces.
Intensity measures: Intensity measures are metrics used to quantify the strength and characteristics of ground shaking during seismic events. These measures help engineers and seismologists assess the potential impact of earthquakes on structures, providing crucial data for seismic hazard analysis and understanding ground motion characteristics. By analyzing intensity measures, engineers can better design structures to withstand seismic forces and minimize damage during earthquakes.
Magnitude-distance pairs: Magnitude-distance pairs are a set of measurements used to describe the relationship between the magnitude of seismic events and their distance from a site of interest. This concept is crucial in seismic hazard analysis as it helps in understanding how ground motion characteristics vary based on the distance from the earthquake source, allowing engineers to better design structures that can withstand seismic forces.
National Seismic Hazard Maps: National seismic hazard maps are graphical representations that depict the likelihood of various levels of ground shaking and associated seismic hazards occurring in different regions over a specified time period. These maps are essential tools for understanding seismic risks, guiding building codes, and informing construction practices, ultimately aiming to enhance safety and resilience against earthquakes.
Near-fault effects: Near-fault effects refer to the unique seismic ground motion characteristics experienced in the vicinity of a fault line during an earthquake. These effects can significantly differ from those observed farther away from the fault, primarily due to complex wave propagation patterns and local site conditions. Understanding near-fault effects is essential for accurate seismic hazard analysis and designing structures that can withstand the intense ground shaking that often occurs near active faults.
NEHRP Site Classes: NEHRP Site Classes are a classification system used to categorize soil and site conditions based on their seismic response characteristics. This system helps engineers and geologists assess how different types of soil can amplify or dampen seismic waves during an earthquake, which is crucial for seismic hazard analysis and understanding ground motion characteristics.
Peak Ground Acceleration: Peak ground acceleration (PGA) is a key measure of ground shaking intensity during an earthquake, quantified as the maximum acceleration experienced by the ground at a specific location. This measure is crucial for assessing the seismic hazard of a region, as it directly influences the design and safety of structures, particularly in areas prone to seismic activity.
Peak Ground Displacement: Peak ground displacement refers to the maximum horizontal or vertical movement experienced by the ground during an earthquake event. This measure is crucial in understanding how seismic waves propagate through the earth and influence structures above, affecting their design and safety considerations.
Peak Ground Velocity: Peak ground velocity (PGV) refers to the maximum speed at which the ground shakes during an earthquake. This measure is crucial as it directly relates to the potential for damage to structures and is used in seismic hazard analysis to evaluate ground motion characteristics that can impact engineering designs and public safety.
Probabilistic seismic hazard analysis: Probabilistic seismic hazard analysis (PSHA) is a method used to evaluate the likelihood of various levels of ground shaking and related ground motion parameters occurring at a site over a specified time frame. This approach takes into account the uncertainty in earthquake occurrence, the behavior of seismic waves as they travel through the earth, and the local geological conditions. By assessing these probabilities, engineers can design structures that are better suited to withstand potential seismic events and meet specific performance objectives.
Return Period: Return period is the average time interval between occurrences of a specific event, such as an earthquake, that exceeds a certain magnitude. This concept is crucial in assessing seismic hazard, as it helps engineers and planners understand the frequency of potentially damaging ground motions. It is often used to determine design criteria for structures by estimating how often an earthquake of a particular size can be expected to occur in a given location.
Seismic design categories: Seismic design categories are classifications that define the seismic risk of buildings and structures based on their location, use, and the potential for ground shaking during an earthquake. These categories help engineers determine the appropriate design requirements and construction practices needed to ensure safety and structural integrity in areas prone to seismic activity, connecting seismic hazard analysis with the expected ground motion characteristics.
Seismic hazard analysis: Seismic hazard analysis is a systematic approach used to evaluate the likelihood and potential impact of earthquake-related ground shaking and other seismic effects at a specific location. This analysis helps in understanding how seismic forces can affect structures and infrastructure, thereby aiding in risk assessment and informed decision-making for engineering and construction practices.
Significant duration: Significant duration refers to the time period over which strong ground shaking occurs during an earthquake, which can greatly influence the response of structures and the level of damage experienced. Understanding significant duration is crucial in seismic hazard analysis as it helps engineers assess the potential impact of earthquakes on different types of structures, informing design decisions and safety measures.
Site Classification Systems: Site classification systems are frameworks used to categorize different types of ground conditions based on their seismic behavior and response during an earthquake. These systems help engineers and planners understand how various soil types and geological formations can influence ground motion characteristics, which is crucial for seismic hazard analysis and design of structures.
Site response analysis: Site response analysis is the process of evaluating how seismic waves propagate through the soil and rock layers at a specific location, influencing the ground motion experienced during an earthquake. This analysis helps in understanding how local soil conditions can amplify or de-amplify seismic waves, affecting structures built on that site. It is crucial for assessing potential ground shaking and designing effective seismic-resistant structures, particularly in areas prone to earthquakes.
Site-specific analyses: Site-specific analyses refer to detailed evaluations conducted at a particular location to assess the unique seismic hazards and ground motion characteristics that may affect structures in that area. These analyses are critical in understanding how local geological conditions, historical seismic activity, and site parameters can influence the behavior of structures during earthquakes. By tailoring assessments to specific sites, engineers can make more informed decisions about design, safety measures, and risk mitigation strategies.
Soil liquefaction potential: Soil liquefaction potential refers to the likelihood of saturated soil to lose its strength and stiffness in response to applied stress, particularly during seismic events. This phenomenon occurs when seismic shaking causes pore water pressure to increase, reducing effective stress and allowing soil particles to behave like a liquid. Understanding this potential is critical in assessing seismic hazards and their effects on structures built on or within susceptible soils.
Soil-structure interaction: Soil-structure interaction refers to the response of soil and structural systems to applied loads, especially during events like earthquakes or heavy traffic. This interaction plays a critical role in understanding how structures behave under seismic forces, as the soil can significantly affect the dynamic response and stability of buildings and bridges. The nature of this interaction can influence design considerations, ground motion characteristics, and overall structural performance.
Spatial variability: Spatial variability refers to the differences in characteristics, properties, or behaviors of a phenomenon across different locations in space. In the context of seismic hazard analysis and ground motion characteristics, understanding spatial variability is crucial for assessing how ground shaking can differ in intensity and duration across a region during an earthquake, impacting building performance and safety.
Spectral Acceleration: Spectral acceleration is a measure used in earthquake engineering to quantify the response of a structure to seismic ground motion at different frequencies. It is derived from the acceleration time history of ground shaking and provides insight into how structures will respond during an earthquake. This concept is critical for understanding the potential impact of seismic events on structures, enabling engineers to design safer buildings and bridges that can withstand earthquakes.
Time-dependent seismic hazard models: Time-dependent seismic hazard models are analytical frameworks that account for the changing likelihood of seismic events over time. These models consider factors such as stress accumulation on faults and historical earthquake records, allowing for more accurate predictions of ground motion characteristics as a function of time. By incorporating temporal changes, these models provide insights into the evolving risk of earthquakes, enhancing the assessment and design of structures in seismically active regions.
Uniform Hazard Spectra: Uniform Hazard Spectra represent the anticipated ground motion at a specific site for various frequencies, based on seismic hazard analysis. These spectra are essential in understanding how different structures will respond to seismic activities, as they provide a means to quantify the likelihood of various levels of ground shaking occurring over a given period.
Vertical Motions: Vertical motions refer to the upward and downward movements of the ground and structures caused by seismic activity. These motions are critical for understanding how buildings and bridges respond during earthquakes, as they can affect structural integrity, load distribution, and overall stability.