Enhancing the Resilience of Pre-1970s Buildings with Shear Wall Retrofitting
Table of Contents
- Introduction
- Background
- Problem Statement
- Types of Shear Wall Failures
- Diagonal Failures
- Flexural Failures
- Lap Splice Bar Failures
- Shear Sliding Failures
- Wall Buckling Failures
- Existing Building Types in California
- Wall Strength of Pre-1970s Buildings
- Research Objectives
- Retrofitting Strategies
- Fiber Reinforced Polymer (FRP)
- Shotcrete
- Experimental Testing
- Control Specimen
- FRP Retrofit Specimen
- Numerical Modeling
- Results and Analysis
- Comparison of Unretrofitted and Retrofitted Specimens
- Future Work
- Acknowledgments
- Conclusion
Introduction 👋
The seismic vulnerability of structures built before the 1970s in California poses a significant threat, especially in the event of a large earthquake. In this article, we will explore the experimental retrofit of non-ductile shear walls and analyze the effectiveness of different retrofitting strategies. By investigating typical damage patterns and evaluating retrofitting methods like fiber reinforced polymer (FRP) and shotcrete, we aim to provide valuable insights into ensuring the safety of these structures.
Background 🏢
Structures constructed in California before the 1970s have outdated seismic standards, leaving them susceptible to failure during seismic events. These structures lack the necessary reinforcement to withstand the forces imposed by earthquakes, putting the lives of occupants at risk. Retrofitting these buildings becomes crucial to enhance their structural integrity and prevent catastrophic damage.
Problem Statement ❗
The main issue lies in the fact that many of these older structures have non-ductile shear walls, which are prone to various failure modes. Common failure modes include diagonal failures, flexural failures, lap splice bar failures, shear sliding failures, and wall buckling failures. It is essential to understand these failure modes and develop effective retrofitting strategies to address them.
Types of Shear Wall Failures 🚧
Diagonal Failures
Diagonal failures occur in reinforced concrete shear walls when excessive tensile and compressive forces are applied diagonally. These failures result in diagonal cracks that propagate along the wall, compromising its integrity.
Flexural Failures
Flexural failures involve the excessive flexing or bending of the shear wall due to inadequate reinforcement. This can lead to cracks and localized damage within the wall, rendering it susceptible to failure.
Lap Splice Bar Failures
Lap splicing, a common construction practice, involves overlapping reinforcing bars for enhanced strength. However, if lap splices are not adequately designed or installed, they can become weak points in the shear wall, leading to failure.
Shear Sliding Failures
Shear sliding failures occur when there is insufficient friction between adjacent components of the shear wall, causing them to slide past each other under seismic forces. This can result in significant damage and compromised structural integrity.
Wall Buckling Failures
Wall buckling failures are characterized by the instability and buckling of the shear wall due to inadequate strength and stiffness. This failure mode is particularly dangerous as it can lead to the collapse of the entire structure.
Existing Building Types in California 🏢
To accurately simulate real-life scenarios, the study focused on existing building types prevalent in the California region. These included:
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"Pylusters" or Barbell Sections: These shear walls feature larger clumps of concrete on the ends, resembling a big I-beam section. The web of these walls has minimal reinforcement (0.2%), while the columns have higher reinforcement ratios (3.5% or more).
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Rectangular Walls with Lap Splices: Lap splicing is commonly used in shear walls, where the reinforcing bars are overlapped to enhance strength. These walls typically have rectangular shapes.
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Flanged Walls: Flanged walls are characterized by a flange-like projection on one side, providing additional resistance against seismic forces.
Wall Strength of Pre-1970s Buildings 💪
The strength of concrete and steel used in pre-1970s buildings was significantly lower compared to current standards. The concrete compressive strength was as low as 3,000 to 4,000 pounds per square inch (PSI), while the steel strength was only around 40,000 PSI. Retrofitting these structures is vital to enhance their strength and ensure their ability to withstand seismic events.
Research Objectives 🎯
The primary objective of this research is to investigate the typical damage patterns that occur during an earthquake event in non-ductile shear walls. Additionally, we aim to evaluate the effectiveness of retrofitting strategies such as fiber reinforced polymer (FRP) and shotcrete. By conducting experimental testing and numerical modeling, we seek to provide valuable insights and design recommendations for safe and cost-effective retrofitting.
Retrofitting Strategies 🔧
Two main retrofitting strategies were considered for this study: fiber reinforced polymer (FRP) and shotcrete.
Fiber Reinforced Polymer (FRP)
FRP involves the application of fiber-reinforced polymer sheets or strips onto the shear wall surface. These sheets are anchored into the structure and enhance its strength and resistance to seismic forces. FRP retrofitting is cost-effective, environmentally friendly, and can significantly improve the structural performance of non-ductile shear walls.
Shotcrete
Shotcrete is a method that involves spraying a mixture of concrete or mortar onto the shear wall surface at high velocity. This application adds an additional layer of material to enhance the strength and integrity of the wall. Shotcrete retrofitting is commonly used in construction and has proven to be an effective method for enhancing structural resilience.
Experimental Testing 🧪
Several specimens were tested to evaluate the effectiveness of different retrofitting strategies. The control specimen represented an unretrofitted shear wall, while the other specimens were retrofitted using FRP and shotcrete.
Control Specimen
The control specimen, representing an unretrofitted shear wall, displayed typical failure patterns observed in non-ductile shear walls during earthquakes. Cracks developed in the web region, gradually increasing in angle and height as the testing progressed. Eventually, diagonal cracks formed across the wall, leading to localized failure and shear crushing in the toe region.
FRP Retrofit Specimen
The FRP retrofit specimen demonstrated a different failure mechanism compared to the control specimen. Rather than developing significant cracks in the web region, the majority of the damage occurred in the toes of the columns. The flexural reinforcement in the columns effectively absorbed the forces, preventing the formation of diagonal cracks. This retrofit strategy showed promising results with higher ductility and strength compared to the unretrofitted specimen.
Numerical Modeling 🔢
To further validate the experimental results, numerical models were developed. Nonlinear beam-truss elements and shell elements were used to simulate the behavior of the shear wall and the FRP overlay, respectively. The models accurately represented the inelastic behavior and failure modes observed during the experimental testing. The numerical results closely matched the experimental force-displacement curves, providing additional confidence in the effectiveness of the retrofitting strategies.
Results and Analysis 📊
A comparison between the unretrofitted and retrofitted specimens revealed significant improvements in terms of ductility and failure modes. While the retrofitted specimens did not exhibit a significant increase in strength, they demonstrated enhanced ductility, which is crucial for allowing time for the occupants to evacuate during seismic events. The strain in the vertical reinforcement and the diagonal peak strain confirmed the effectiveness of the retrofitting strategies in redistributing forces and preventing shear failures.
Future Work 🔍
The ongoing research includes the testing of a shotcrete retrofit specimen. By comparing the performance of different retrofitting strategies, a comprehensive understanding of their effectiveness can be achieved. The results will further contribute to the development of design recommendations for safe and cost-effective retrofitting methods for non-ductile shear walls.
Acknowledgments 🙏
This research project was made possible through the support and sponsorship of the National Institute for Standards and Technology. We would like to extend our gratitude to Simpson Strong-Tie for donating the materials used in this study. Special thanks to our principal investigators, Dr. Ganos and Dr. Juan Marcio Delo, for their guidance and expertise. We would also like to express our appreciation to our advisory committee, whose valuable insights have been instrumental in the success of this project.
Conclusion 🏁
The experimental retrofit of non-ductile shear walls provides valuable insights into enhancing the structural resilience of pre-1970s buildings. By utilizing retrofitting strategies such as fiber reinforced polymer (FRP) and shotcrete, it is possible to improve the ductility and failure modes of these structures. Through further research and development, we can provide design recommendations that ensure the safety and longevity of non-ductile shear walls in seismic-prone regions.
Highlights ✨
- The seismic vulnerability of pre-1970s buildings in California poses a significant threat during earthquakes.
- Non-ductile shear walls in these structures are susceptible to different failure modes, including diagonal failures, flexural failures, lap splice bar failures, shear sliding failures, and wall buckling failures.
- Retrofitting strategies such as fiber reinforced polymer (FRP) and shotcrete can enhance the strength and resilience of non-ductile shear walls.
- Experimental testing and numerical modeling provide valuable insights into the performance and effectiveness of retrofitting methods.
- FRP retrofitting demonstrates improved ductility and failure modes compared to unretrofitted shear walls.
- The ongoing research includes the testing of a shotcrete retrofit specimen to further evaluate retrofitting strategies.
- Design recommendations can be developed to ensure the safe and cost-effective retrofitting of non-ductile shear walls.
FAQs 🤔
Q: Why are pre-1970s buildings in California at a higher risk during earthquakes?
A: Pre-1970s buildings in California were constructed before current seismic standards were in place. These buildings lack the necessary reinforcement to withstand the forces generated by earthquakes, making them more susceptible to damage and collapse during seismic events.
Q: What are the common failure modes of non-ductile shear walls?
A: Common failure modes of non-ductile shear walls include diagonal failures, flexural failures, lap splice bar failures, shear sliding failures, and wall buckling failures. These failure modes can lead to cracks, localized damage, and compromised structural integrity.
Q: How does fiber reinforced polymer (FRP) retrofitting improve the performance of non-ductile shear walls?
A: FRP retrofitting involves the application of fiber reinforced polymer sheets or strips onto the shear wall surface. This reinforcement enhances the strength and resistance of the wall, redistributing forces and preventing shear failures. FRP retrofitting is cost-effective, environmentally friendly, and can significantly improve the structural performance of non-ductile shear walls.
Q: What can be expected from future research on retrofitting non-ductile shear walls?
A: Future research will include testing a shotcrete retrofit specimen to compare the performance of different retrofitting strategies. This research aims to provide comprehensive insights into the effectiveness of retrofitting methods and contribute to the development of design recommendations for safe and cost-effective retrofitting of non-ductile shear walls.
Q: How can the results of this research contribute to the field of structural engineering?
A: The results of this research provide valuable insights into retrofitting strategies for non-ductile shear walls. By understanding the failure modes and effectiveness of retrofitting methods, structural engineers can make informed decisions and recommendations to enhance the safety and resilience of structures in seismic-prone regions.
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