9.2 Retrofit strategies for various structural systems
2 min read•july 25, 2024
Seismic retrofitting is crucial for upgrading existing structures to withstand earthquakes. Global strategies address system-wide issues, while local strategies target specific elements. The choice depends on the building's vulnerabilities and desired performance level.
Retrofit design varies by material, with concrete structures benefiting from and FRP wrapping, from bracing, and masonry from . Effectiveness is evaluated through performance-based methods, , and , with ensuring real-world applicability.
Global and Local Retrofit Strategies
Global vs local retrofit strategies
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- Global retrofit strategies affect overall structural behavior addressing system-wide deficiencies through adding , installing systems, or implementing damping devices
- Local retrofit strategies target specific elements addressing localized weaknesses via column jacketing, beam-column joint strengthening, or infill wall reinforcement
Selection of retrofit techniques
- structures benefit from shear wall addition increasing lateral stiffness, column confinement improving , and beam-column joint strengthening
- Steel structures enhanced by boosting lateral resistance, , and connection upgrades
- Masonry structures strengthened through surface treatments (shotcrete, FRP wrapping), , and
- Timber structures improved by , , and shear wall implementations
Retrofit Design and Evaluation
Retrofit design for structural systems
- Reinforced concrete retrofit design involves concrete jacketing to increase column cross-section and add reinforcement, and FRP wrapping to enhance flexural and shear capacity
- Steel structure retrofit design utilizes to increase energy dissipation and lateral stiffness, and (BRBs) to improve ductility
- Masonry structure retrofit design employs drilling vertical cores filled with reinforced grout, and inserting horizontal reinforcement in mortar joints
Effectiveness of seismic retrofits
- Performance-based evaluation methods include:
- assessing capacity curve changes
- comparing pre- and post-retrofit responses
- Fragility curve analysis develops curves for retrofitted structures comparing probability of damage states
- Cost-benefit analysis evaluates initial retrofit costs, assesses potential loss reduction, and considers life-cycle costs
- Experimental validation uses of retrofitted components and full-scale testing of retrofitted structures
Key Terms to Review (37)
Anchor bolt additions: Anchor bolt additions refer to the process of installing new anchor bolts or enhancing existing ones in a structure to improve its stability and resistance against seismic forces. These bolts are crucial for connecting structural elements, such as columns and walls, to the foundation, especially during retrofitting efforts aimed at enhancing a building's performance during earthquakes.
ASCE 41: ASCE 41 is a standard developed by the American Society of Civil Engineers that provides guidelines for the seismic rehabilitation of existing buildings. This standard is crucial as it outlines procedures and criteria for evaluating and retrofitting structures to improve their earthquake performance, ensuring safety and resilience against seismic events. The standard includes methods like nonlinear static procedures, which are essential for understanding how buildings respond to earthquakes and determining effective retrofit strategies.
Base Isolation: Base isolation is a seismic design technique that allows a building to move independently from ground motion during an earthquake, effectively reducing the forces transmitted to the structure. This technique enhances the resilience of buildings, making them safer in seismic-prone areas by decoupling the building from ground vibrations, which is essential for various aspects of seismic design.
Bed joint reinforcement: Bed joint reinforcement refers to the use of steel reinforcement bars or mesh placed horizontally within the mortar bed joints of masonry walls to enhance their structural integrity and resistance to lateral forces. This technique is particularly beneficial in improving the overall performance of masonry structures during seismic events, as it helps to distribute loads and prevent cracking or failure. By incorporating bed joint reinforcement, engineers can significantly increase the durability and safety of masonry buildings, making them more resilient to earthquakes and other external stresses.
Bracing Systems: Bracing systems are structural components used in buildings to resist lateral forces, such as those caused by wind and earthquakes. These systems help to maintain the stability and integrity of a structure by providing support and preventing deformation during seismic events. Bracing can take various forms, including diagonal braces, shear walls, and moment-resisting frames, each tailored to enhance the resilience of a structure against dynamic loads.
Buckling-restrained braces: Buckling-restrained braces (BRBs) are structural elements designed to provide lateral stability to buildings during seismic events by absorbing and dissipating energy. These braces prevent buckling under compression by using a specially designed casing that allows the core material to yield while remaining stable, thus improving the overall resilience of structures. The use of BRBs enhances the effectiveness of supplemental damping systems and plays a significant role in retrofitting existing buildings to meet modern seismic standards.
Center core technique: The center core technique is a seismic retrofit strategy that involves the addition of a reinforced core within an existing building structure to enhance its lateral stability and strength during earthquakes. This approach not only increases the overall performance of the structure but also helps to redistribute forces and reduce deformation, making it particularly effective for various structural systems that may be vulnerable to seismic activities.
Cost-benefit analysis: Cost-benefit analysis is a systematic process used to evaluate the economic feasibility of projects by comparing the expected costs against the anticipated benefits. This method helps in making informed decisions about investments, especially in areas like disaster preparedness and risk management. By quantifying potential losses from hazards and weighing them against mitigation costs, stakeholders can prioritize strategies that offer the best economic returns while minimizing risks.
Crack injection with grouting: Crack injection with grouting is a repair technique used to fill and seal cracks in concrete and masonry structures by injecting a liquid material, usually a type of grout, into the cracks. This method is crucial for enhancing the structural integrity of buildings, particularly in seismic retrofitting, as it helps to restore load-bearing capacity and prevent further deterioration.
Diaphragm strengthening: Diaphragm strengthening refers to the process of reinforcing the horizontal structural elements of a building, such as floors and roofs, to improve their ability to resist lateral forces during events like earthquakes. This method is crucial for enhancing a structure's overall stability and ensuring that it can effectively distribute seismic loads across its frame. By upgrading diaphragms, buildings can maintain better integrity under stress, ultimately protecting occupants and property.
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.
Eccentric bracing: Eccentric bracing is a structural system used to improve the lateral load resistance of buildings during seismic events by utilizing diagonal braces that are not aligned with the vertical columns. This approach allows for more effective energy dissipation and stiffness, reducing the overall movement of the structure during an earthquake. By strategically placing these braces, eccentric bracing systems can enhance the performance of a building, making them a popular choice for retrofitting existing structures to meet modern seismic codes.
Energy dissipation devices: Energy dissipation devices are specialized systems or components used in structures to absorb and dissipate energy, particularly during dynamic events such as earthquakes. These devices play a crucial role in reducing the amount of energy transmitted to the building, thereby enhancing its overall stability and safety. By converting kinetic energy into other forms, these devices mitigate damage and improve the performance of structures subjected to seismic forces.
Experimental validation: Experimental validation is the process of verifying the accuracy and reliability of theoretical models or design approaches through physical testing and observation. This concept is crucial in ensuring that engineered structures can perform as intended during seismic events, providing insights into how design methodologies translate to real-world performance. It links theoretical predictions with actual behavior, enhancing our understanding of how structures respond to forces such as earthquakes.
F. noe: In the context of earthquake engineering, 'f. noe' refers to the nonlinear response factor, a crucial parameter in assessing how structures behave during seismic events. It helps engineers understand how a structure's performance might differ from its expected linear behavior under large deformations caused by earthquakes, allowing for more accurate design and retrofit strategies.
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.
Flexibility: Flexibility refers to the ability of a structural system to deform or bend under applied loads without collapsing. In seismic design, flexibility is crucial because it allows buildings to absorb and dissipate energy during an earthquake, reducing the potential for damage. This adaptability is essential in various contexts, including how moment-resisting frames are designed, how mechanical and electrical systems are protected, and how retrofitting strategies are applied to older structures.
Fragility Curves: Fragility curves are graphical representations that show the probability of reaching or exceeding a specific level of damage to a structure given a certain level of seismic demand. These curves help in understanding how different structures respond to earthquakes, making them essential for assessing the vulnerability of buildings and infrastructure. By linking seismic hazard data with potential structural responses, fragility curves play a critical role in risk assessment, damage prediction, and informing retrofitting strategies.
George W. Housner: George W. Housner was a prominent American civil engineer and researcher known for his contributions to earthquake engineering and structural dynamics. He developed influential theories and methodologies that significantly advanced the understanding of how structures respond to seismic forces, particularly through his work on dynamic analysis and the concept of response spectra, which became essential tools in the design and retrofit of structures for earthquake resilience.
IBC: The International Building Code (IBC) is a set of codes established to provide minimum standards for building safety and structural integrity, addressing various aspects of construction including seismic design. It serves as a vital guideline to ensure that structures can withstand the forces of nature, such as earthquakes, making it essential for engineers in their design processes.
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.
Load Path Analysis: Load path analysis is a method used to understand how forces and loads are transferred through a structure. This analysis identifies the routes that loads take from the point of application, through various structural components, until they reach the foundation. Understanding load paths is crucial in retrofitting strategies, as it helps engineers ensure that modifications to a structure will maintain its integrity under different load conditions.
Moment frame modifications: Moment frame modifications refer to the enhancements made to existing moment-resisting frames in buildings to improve their performance during seismic events. These modifications can involve retrofitting techniques that strengthen the connections and members of the frame, allowing it to better withstand lateral forces generated by earthquakes. Effective moment frame modifications are crucial for ensuring the safety and resilience of structures, especially in seismic-prone regions.
Performance Levels: Performance levels refer to the defined thresholds that indicate how well a structure or system can withstand various levels of seismic activity, typically categorized from minor to major events. These levels help in assessing a structure's ability to perform safely under different earthquake scenarios and guide the decision-making process for retrofitting strategies aimed at improving structural resilience.
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 concrete: Reinforced concrete is a composite building material that combines concrete's compressive strength with the tensile strength of steel reinforcement bars, or rebar. This combination allows structures to withstand greater loads and resist cracking under stress, making it ideal for use in seismic design and retrofitting strategies.
Seismic performance: Seismic performance refers to how a structure behaves during and after an earthquake, focusing on its ability to withstand ground shaking and avoid collapse. This concept is crucial in understanding the effectiveness of various design strategies, like base isolation systems and retrofit techniques, which aim to improve a building's resilience against seismic events.
Shake table testing: Shake table testing is a method used to simulate the effects of earthquakes on structures by placing a model of the structure on a large platform that can move in various directions. This technique helps engineers understand how different structural designs will perform during seismic events, allowing for improvements in safety and resilience. The insights gained from these tests are crucial in developing retrofit strategies for existing buildings and designing new ones to withstand earthquakes.
Shear Walls: Shear walls are structural elements designed to resist lateral forces, primarily from wind or earthquakes, by providing stiffness and strength to a building. These walls are typically vertical and made from reinforced concrete or masonry, allowing them to effectively transfer horizontal loads down to the foundation. Their role is crucial in multi-degree-of-freedom systems, where they enhance stability and reduce sway during seismic events. Additionally, shear walls can be analyzed under nonlinear dynamic analysis to assess their performance during extreme loading scenarios, making them essential for retrofit strategies in existing structures.
Steel braces: Steel braces are structural elements used in buildings and other constructions to enhance stability and resistance against lateral forces, particularly during seismic events. These braces help to distribute loads and prevent structural failure by creating a rigid frame that can absorb and dissipate energy during an earthquake, thus protecting both mechanical and electrical systems and overall structural integrity.
Steel structures: Steel structures are constructions made primarily from steel components, which provide strength, durability, and flexibility. They are widely used in buildings, bridges, and various infrastructure due to their ability to withstand high loads and resist environmental impacts. Understanding retrofit strategies for these structures is crucial, especially in the context of improving resilience against seismic events.
Surface treatments: Surface treatments refer to various techniques applied to the exterior of structural elements to enhance their performance, durability, and resistance to environmental factors. These treatments can improve seismic resilience by providing better energy dissipation and reducing damage during earthquakes. They are crucial in retrofit strategies as they enable older structures to meet modern safety standards and requirements.
Through-wall anchors: Through-wall anchors are structural elements used to connect walls and provide lateral support by transferring loads across the thickness of the wall. They play a vital role in ensuring the stability of various structural systems, particularly during seismic events, by anchoring different components together and preventing movement or separation. Their proper installation and design can significantly enhance the seismic performance of a structure.
Time History Analysis: Time history analysis is a method used in structural engineering to assess how a structure responds over time to specific loading conditions, typically seismic events. This approach allows engineers to capture the dynamic behavior of structures under realistic earthquake ground motions, which can vary in amplitude and frequency. By analyzing how a structure reacts at each point in time, this method provides crucial insights for the design and evaluation of buildings and infrastructure in earthquake-prone areas.
Tuned mass damper: A tuned mass damper is a device used in structures to reduce vibrations caused by external forces like earthquakes or strong winds. It works by utilizing a mass that is tuned to oscillate out of phase with the vibrations of the building, effectively canceling them out. This is especially important in retrofitting existing structures, as it enhances their ability to withstand dynamic loads without major modifications to the original design.
Viscoelastic damping: Viscoelastic damping refers to the energy dissipation mechanism in materials that exhibit both viscous and elastic properties when subjected to stress. This type of damping is critical in mitigating vibrations and reducing dynamic response in structures during events like earthquakes, making it a vital consideration for enhancing the performance of various structural systems.
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.