13.4 Seismic design of foundations
3 min read•august 16, 2024
Earthquakes can wreak havoc on buildings, but proper foundation design can help structures withstand the shaking. This section dives into how engineers create foundations that can handle seismic forces, considering and site-specific hazards.
From shallow footings to deep piles, different foundation types behave uniquely during earthquakes. We'll explore how soil properties, liquefaction risks, and design techniques like can make or break a building's ability to stay standing when the ground starts moving.
Seismic Foundation Design Principles
Soil-Structure Interaction in Seismic Design
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- Seismic foundation design creates structures withstanding earthquake-induced ground motions and minimizing superstructure damage
- Soil-structure interaction (SSI) forms dynamic relationship between foundation, soil, and superstructure during seismic events
- Design principles consider site-specific seismic hazards, soil properties, and structural characteristics
- Inertial interaction effects cause additional forces and deformations in soil-foundation system from structure and foundation inertia
- Kinematic interaction effects lead to foundation input motions differing from free-field ground motions
- Result from motion differences between free-field soil and foundation
Seismic Design Considerations
- Account for potential soil liquefaction, lateral spreading, and slope instability induced by earthquake shaking
- Selection of appropriate foundation systems and design parameters mitigates seismic risks
- Ensure structural integrity during earthquakes through proper foundation design
- Consider soil bearing capacity, settlement potential, and lateral earth pressures for shallow foundations
- Evaluate deep foundations' ability to transfer loads to deeper, more stable soil layers
- Assess impact of soil conditions (soil type, density, groundwater level) on seismic response of foundations
- Analyze liquefaction-susceptible soils for potential bearing capacity failure or excessive settlement
Foundation Performance in Seismic Conditions
Shallow vs. Deep Foundations
- Shallow foundations (spread footings, mat foundations) behave differently under seismic loads compared to deep foundations (piles, caissons)
- Deep foundations generally provide better seismic resistance
- Transfer loads to deeper, more stable soil layers
- Shallow foundation performance influenced by:
- Soil bearing capacity
- Settlement potential
- Lateral earth pressures during earthquakes
- Deep foundation advantages:
- Increased stability
- Reduced susceptibility to surface soil effects
- Better performance in liquefiable soils
Soil Properties and Seismic Response
- Dynamic soil properties crucial in determining seismic response of soil-foundation systems
- Shear modulus
- Damping ratio
- Liquefaction-susceptible soils pose challenges for foundation performance
- Potential for bearing capacity failure
- Risk of excessive settlement
- Soil conditions significantly affect seismic response of foundations
- Soil type (clay, sand, rock)
- Soil density
- Groundwater level
- Numerical modeling techniques evaluate seismic performance under various soil conditions
- Finite element analysis
- Lumped parameter models
Seismic Foundation Design for Resistance
Design Parameters and Approach
- Determine design earthquake parameters
- Peak ground acceleration
- Response spectra
- Design ground motions
- Implement capacity design approach
- Ensures foundations remain elastic
- Allows controlled inelastic behavior in superstructure during severe earthquakes
- Account for static and seismic load combinations in foundation sizing and reinforcement detailing
- Overturning moments
- Conduct geotechnical investigations and site-specific seismic hazard analyses
- Determine appropriate design parameters
- Assess soil properties
Mitigation Techniques and Considerations
- Design for ductility in foundation elements to dissipate seismic energy and prevent brittle failure modes
- Implement soil improvement methods for liquefiable soils
- Deep soil mixing
- Stone columns
- Dynamic compaction
- Consider potential permanent ground deformations in foundation design
- Lateral spreading
- Slope instability during earthquakes
- Evaluate foundation performance under different soil conditions
- Cohesive soils (clay)
- Granular soils (sand)
- Bedrock
Seismic Isolation and Energy Dissipation in Foundations
Seismic Isolation Techniques
- Seismic isolation systems decouple superstructure from ground motion
- Reduce seismic forces transmitted to structure
- Base isolation techniques commonly used in foundation design
- Elastomeric bearings
- Friction pendulum systems
- Design isolator-foundation interface for proper load transfer
- Vertical load transfer
- Horizontal load transfer
- Accommodate large displacements in seismic isolation systems
- Maintain stability
- Prevent pounding between isolated and non-isolated components
Energy Dissipation Devices
- Incorporate devices into foundation systems
- Viscous dampers
- Hysteretic dampers
- Effectiveness of seismic isolation and energy dissipation depends on:
- Soil conditions
- Structural characteristics
- Seismic hazard level
- Implement performance-based design approaches
- Optimize seismic isolation and energy dissipation systems
- Tailor designs to specific project requirements
- Account for unique site conditions
Key Terms to Review (18)
ASCE 7: ASCE 7 is a standard published by the American Society of Civil Engineers that provides minimum design loads for buildings and other structures. It serves as a crucial reference for engineers in assessing and designing structures to withstand various forces, including seismic, wind, and snow loads, ensuring safety and reliability in construction.
Base isolation: Base isolation is a seismic design technique that aims to reduce the amount of energy transferred from ground motion to a structure during an earthquake by decoupling the building from ground vibrations. This is achieved through the use of flexible bearings or isolators that allow the building to move independently from the ground motion, significantly minimizing structural damage and improving safety. By incorporating base isolation, engineers can enhance the resilience of buildings, particularly in earthquake-prone regions.
Dynamic analysis: Dynamic analysis refers to the assessment of structures and materials under time-dependent loads, particularly those caused by seismic events. This method evaluates how structures respond to dynamic forces like earthquakes, which is crucial for ensuring their stability and safety. By understanding the dynamic behavior of slopes and retaining walls during seismic activities, engineers can design more resilient structures that withstand these forces effectively.
Energy Dissipation: Energy dissipation refers to the process through which energy is absorbed and transformed into a less useful form, typically heat, during dynamic loading events such as earthquakes. In the context of foundations subjected to seismic forces, energy dissipation is crucial as it helps to reduce the amplitude of vibrations and minimize structural damage. This concept is essential for designing foundations that can effectively manage seismic loads and protect both structures and occupants from the devastating impacts of earthquakes.
Finite Element Method: The Finite Element Method (FEM) is a numerical technique used for solving complex engineering and mathematical problems by breaking down a larger system into smaller, simpler parts called finite elements. This method is particularly useful in analyzing physical phenomena such as seepage, stress distribution, and slope stability, allowing engineers to predict how structures will respond under various conditions.
IBC: IBC stands for International Building Code, a set of regulations developed to establish minimum safety and performance standards for buildings and structures. It serves as a comprehensive framework that addresses design and construction practices, ensuring public health, safety, and welfare. The IBC provides guidelines relevant to various aspects of foundation design, including those for shallow foundations and considerations necessary for seismic events, which help prevent structural failures during such occurrences.
Inertia forces: Inertia forces are forces that arise from an object's resistance to changes in its state of motion, especially when subjected to acceleration or deceleration. These forces play a significant role in the seismic design of foundations, as they help determine how structures respond to ground shaking during an earthquake. Understanding inertia forces is crucial for engineers to ensure that foundations can adequately support structures and withstand the dynamic loads imposed by seismic events.
Liquefaction potential: Liquefaction potential refers to the likelihood that saturated soil will lose its strength and stiffness in response to applied stress, typically due to seismic activity, causing it to behave like a liquid. This phenomenon can lead to catastrophic failures in structures and foundations during earthquakes, highlighting its importance in understanding soil behavior and engineering practices related to seismic events.
M. J. N. Priestley: M. J. N. Priestley is a prominent figure in geotechnical engineering, particularly known for his contributions to the seismic design of foundations. His work has greatly influenced the understanding of how soil and structures interact during seismic events, emphasizing the importance of foundation design in earthquake-prone areas. Priestley's theories and methodologies are crucial for ensuring the stability and safety of structures when subjected to seismic forces.
Pier foundation: A pier foundation is a deep foundation type that transmits structural loads to the underlying soil or rock through vertical elements called piers. These piers are typically made of reinforced concrete or steel and extend deep into the ground, providing stability and support for structures, especially in areas with weak or unstable soil. Piers are essential in seismic design as they can accommodate lateral forces and provide greater resilience against ground movement during earthquakes.
Raft foundation: A raft foundation is a large concrete slab that supports a building or structure by distributing its load over a wide area of soil. This type of foundation is particularly useful in situations where soil conditions are poor or variable, as it helps prevent differential settlement and provides stability during seismic events.
Response Spectrum Analysis: Response spectrum analysis is a method used in seismic engineering to estimate the expected response of structures subjected to earthquake ground motions. This technique helps engineers understand how different frequencies of vibrations affect structures, enabling them to design foundations and buildings that can withstand seismic activity. By analyzing the response of a structure to various seismic inputs, it becomes possible to create designs that enhance safety and performance during earthquakes.
Seismic zoning: Seismic zoning refers to the process of categorizing regions based on their expected seismic activity, providing guidelines for the design and construction of structures to withstand earthquakes. It involves assessing the geological and seismic hazards specific to an area, which influences building codes and foundation design. Understanding seismic zoning is essential for engineers and architects to create safe buildings in earthquake-prone regions.
Shaking table testing: Shaking table testing is a dynamic testing method used to simulate the effects of earthquake forces on structures, materials, and foundations. This technique allows engineers to observe how various designs respond to seismic activity, helping to enhance the safety and performance of structures in earthquake-prone areas. By replicating ground motion and assessing the structural response, shaking table testing plays a crucial role in seismic design and analysis.
Shear Forces: Shear forces are internal forces that act parallel to the surface of a material, causing one part of the material to slide past another. In the context of seismic design, understanding shear forces is crucial for assessing how structures respond to lateral loads during an earthquake, as these forces can lead to failure if not properly accounted for in foundation design and structural integrity.
Site amplification: Site amplification refers to the phenomenon where seismic waves are intensified as they pass through specific geological materials and conditions at a site. This effect can significantly increase the ground motion experienced during an earthquake, depending on the local soil properties and the underlying geology. Understanding site amplification is crucial for designing foundations that can withstand amplified seismic forces.
Soil-structure interaction: Soil-structure interaction refers to the complex relationship between the behavior of soil and the structures built upon or within it. This interaction influences how forces, deformations, and vibrations are transferred between the soil and the structure, impacting the overall performance and stability of civil engineering projects. Understanding this relationship is essential for effective foundation design, ensuring that structures can safely withstand environmental loads, such as seismic events.
T. Paulay: T. Paulay is a prominent figure in the field of geotechnical engineering, particularly known for his contributions to the seismic design of foundations. His work emphasizes the importance of understanding soil behavior during seismic events and how it affects the stability and performance of foundations in earthquake-prone areas. Paulay's research has significantly influenced modern practices in seismic design, particularly regarding the interaction between structures and the underlying soil.