9.3 Strengthening techniques for foundations and superstructures
3 min read•july 25, 2024
Earthquakes can wreak havoc on buildings with weak foundations and structures. Inadequate bearing capacity, poor soil conditions, and liquefaction are just a few issues that can lead to catastrophic failure during seismic events.
Luckily, there are ways to strengthen foundations and superstructures. , , and can reinforce foundations, while , , and can boost a building's ability to withstand earthquakes. These methods can significantly improve a structure's seismic performance.
Foundation Seismic Deficiencies and Strengthening
Seismic deficiencies in structures
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- Foundation deficiencies
- Inadequate bearing capacity leads to excessive settlement or failure under seismic loads
- Insufficient lateral resistance causes sliding or overturning during earthquakes
- Poor soil conditions amplify ground motions and increase structural damage (soft clays, loose sands)
- Liquefaction susceptibility results in loss of soil strength and support (saturated sandy soils)
- Superstructure deficiencies
- Weak column-strong beam configurations promote story collapse mechanisms
- Soft story mechanisms concentrate deformation and damage in a single level (open ground floors)
- Inadequate lateral force-resisting systems fail to withstand seismic forces (insufficient shear walls)
- Poor connection details lead to premature failure and loss of load paths (beam-column joints)
- Torsional irregularities cause excessive twisting and uneven distribution of seismic forces
Foundation strengthening techniques
- Underpinning
- Excavation and extension of existing foundations increases depth and bearing capacity
- Mass concrete underpinning provides additional support and stability to shallow foundations
- Beam and base underpinning creates a new foundation system beneath the existing structure
- Micropiles
- Small-diameter reinforced piles offer high-capacity foundation support in limited access areas
- Installation through existing foundations strengthens deep foundation systems
- Suitable for retrofitting historic structures with minimal disturbance
- Jet grouting
- High-pressure injection of grout into soil creates soil-cement columns
- Improves soil strength and stiffness reducing liquefaction potential
- Forms underground structural elements for additional foundation support
Superstructure Strengthening and Performance Assessment
Superstructure reinforcement methods
- Jacketing
- Reinforced concrete jacketing increases member size and reinforcement for enhanced strength and
- Steel jacketing provides confinement and shear strengthening to columns and beams
- (FRP) jacketing offers lightweight, corrosion-resistant strengthening option
- Wing walls
- Addition of reinforced concrete walls perpendicular to existing walls increases lateral stiffness and strength
- Improves overall structural stability and reduces torsional effects
- Effective for strengthening corner columns and enhancing frame action
- Infill walls
- Addition of new walls within existing frame bays improves lateral load resistance
- Masonry or reinforced concrete infills increase overall structural stiffness
- Strategically placed infills can address soft story issues and irregularities
Impact of seismic strengthening
- Performance evaluation methods
- assesses nonlinear behavior and collapse mechanisms
- simulates dynamic response to specific ground motions
- compares structural capacity with seismic demand
- Key performance indicators
- measures overall lateral strength of the structure
- indicate deformation levels and potential for damage
- Ductility quantifies ability to undergo inelastic deformations without failure
- evaluates structure's capacity to absorb seismic energy
- Comparative analysis
- Pre-strengthening vs post-strengthening behavior highlights improvement in seismic response
- Improvement in fundamental period indicates increased stiffness and reduced vulnerability
- Reduction in seismic demand demonstrates effectiveness of strengthening measures
- Cost-benefit analysis
- Strengthening costs vs improved performance guides decision-making process
- Life-cycle cost considerations account for long-term benefits of seismic upgrades
- Risk reduction assessment quantifies decrease in potential losses and casualties
Key Terms to Review (19)
Base Shear Capacity: Base shear capacity refers to the maximum lateral force that a building or structure can withstand at its base during seismic events. This capacity is critical for ensuring the structural integrity and safety of buildings when subjected to ground motion, making it a vital component in the design and evaluation of structures in earthquake-prone areas.
Capacity Spectrum Method: The capacity spectrum method is a seismic analysis technique that combines the capacity curve of a structure with the demand spectrum derived from seismic hazard assessments. This method allows engineers to evaluate how a structure will respond to seismic forces by comparing its inherent strength and ductility against the expected ground motion demands. By using this approach, engineers can identify potential vulnerabilities and make informed decisions on how to enhance the building’s performance during earthquakes.
Drift Ratios: Drift ratios are a measure of the relative lateral displacement between different parts of a structure under load, typically expressed as a percentage of the height of the building. They are crucial in understanding how structures respond to lateral forces, such as those generated by earthquakes. Drift ratios help engineers assess the performance of buildings and their components, ensuring that they can withstand seismic events without experiencing excessive deformation that could lead to structural failure.
Ductility: Ductility is the ability of a material to deform plastically before fracture, allowing structures to absorb and dissipate energy during seismic events. This property is crucial for maintaining structural integrity and safety, as it enables buildings to withstand the forces generated by earthquakes without collapsing instantly.
Dynamic analysis: Dynamic analysis is a method used in engineering to evaluate the response of structures under time-varying loads, such as those caused by earthquakes. This approach helps to predict how a building or bridge will behave during seismic events, providing critical insights for safety and performance. By incorporating dynamic effects, this analysis supports the design process, ensuring that structures can withstand not just static loads but also the unpredictable nature of dynamic forces.
Energy dissipation: Energy dissipation refers to the process by which energy is transformed into a less usable form, often through mechanisms such as damping and friction, during dynamic loading events like earthquakes. This process plays a crucial role in structural engineering by helping to absorb and mitigate the forces exerted on structures, ensuring their stability and integrity during seismic events.
Fiber-reinforced polymer: Fiber-reinforced polymer (FRP) is a composite material made of a polymer matrix reinforced with fibers, typically glass, carbon, or aramid. This combination enhances the material's strength, stiffness, and durability, making it suitable for use in strengthening existing structures and retrofitting various systems. FRP can be applied to foundations and superstructures to improve their load-bearing capacity and resistance to seismic forces.
High-strength concrete: High-strength concrete is a type of concrete that has a compressive strength greater than 40 MPa (megapascals) and is formulated using specific materials and techniques to achieve enhanced durability and performance. This kind of concrete is essential in modern construction, particularly for structures that require superior load-bearing capacity and resistance to various environmental factors.
Infill walls: Infill walls are non-structural walls that are typically made of materials like brick, concrete, or gypsum board, and are used to fill the spaces between structural frames in buildings. While they primarily serve as partitions or enclosures, they also contribute to the overall stability and performance of the building during events like earthquakes, impacting how forces are distributed throughout the structure.
Jacketing: Jacketing is a strengthening technique used to enhance the structural capacity of existing foundations and superstructures by adding an external layer of material, such as concrete or steel, around the original structure. This method effectively increases the strength, stiffness, and ductility of a building or component, making it more resistant to forces like those experienced during earthquakes. It is often applied to retrofit and upgrade various structural systems to improve their performance under seismic loading.
Jet grouting: Jet grouting is a ground improvement technique that involves injecting a cement-based grout into the soil at high pressure to create a solidified mass, enhancing the soil's strength and stability. This method is particularly useful for reinforcing foundations and superstructures, as it can be tailored to specific site conditions and engineering requirements, ensuring better load-bearing capacity and reducing settlement risks.
Micropiles: Micropiles are small-diameter, high-strength structural elements used to support foundations and resist loads in weak or unstable soils. These deep foundation systems are particularly useful in strengthening existing structures, improving stability, and providing support in challenging geotechnical conditions. Their versatility makes them an effective choice for both new construction and retrofitting projects.
Pushover analysis: Pushover analysis is a nonlinear static analysis method used to evaluate the seismic performance of structures by applying a gradual lateral load until failure occurs. This technique helps engineers understand how a structure will respond to seismic forces, identifying potential weaknesses and assessing ductility, which is essential for effective seismic design.
Reinforced steel: Reinforced steel is a composite material made from concrete and steel, where steel bars or mesh are embedded within concrete to improve its tensile strength. This combination allows structures to better withstand various forces, particularly during events like earthquakes, where tension and compression forces can lead to structural failure. The use of reinforced steel is a key technique in enhancing the strength and durability of foundations and superstructures in civil engineering.
Seismic sensors: Seismic sensors are devices used to detect and measure ground motion caused by seismic waves generated from earthquakes or other sources. These sensors play a crucial role in monitoring seismic activity, providing essential data for earthquake early warning systems and aiding in the design of structures by assessing how they respond to ground shaking.
Time-History Analysis: Time-history analysis is a method used in earthquake engineering to assess the dynamic response of structures to seismic events by applying recorded ground motion data over time. This approach allows engineers to capture the effects of varying intensity and frequency of seismic forces on structures, providing insights into their behavior during actual earthquake scenarios.
Underpinning: Underpinning is a construction technique used to strengthen and stabilize the foundations of existing structures by extending or deepening their foundation systems. This method is crucial when the original foundation is inadequate to support the load of a structure, especially in cases where additional stories are added or when ground conditions change. It ensures that the structural integrity is maintained and helps prevent issues like settlement or cracking.
Vulnerability assessment: A vulnerability assessment is a systematic evaluation of a structure or community's susceptibility to damage from seismic events. This process identifies weaknesses and potential points of failure in existing designs, enabling decision-makers to prioritize necessary improvements and implement effective risk reduction strategies. Understanding vulnerabilities plays a crucial role in enhancing safety through strengthening techniques, loss estimations, and developing comprehensive risk mitigation measures.
Wing walls: Wing walls are structural extensions of a bridge or retaining wall that provide lateral support and stability, preventing soil erosion and ensuring the integrity of the structure. These walls extend from the main structure and help to direct water flow, protect against soil movement, and enhance the overall load-bearing capacity of foundations and superstructures.