Renforcement des murs de cisaillement en béton armé: étude expérimentale et numérique

Table of Contents:

1. Introduction
2. Motivation for the Project
3. Failure Modes of Reinforced Concrete Shear Walls 3.1 Diagonal Failures 3.2 Flexal Failures 3.3 Lap Splice Bar Failures 3.4 Shear Sliding Failures 3.5 Out-of-Plane Wall Buckling
4. Existing Building Types in California 4.1 Pylisters or Barbell Sections 4.2 Rectangular Walls with Lap Splices 4.3 Flanged Walls
5. Typical Material Properties 5.1 Concrete Strength 5.2 Steel Strength
6. Objectives of the Research
7. Retrofitting Strategies 7.1 Fiber Reinforced Polymer (FRP) 7.2 Shotcrete
8. Experimental Testing and Results 8.1 Control Specimen 8.2 FRP Specimen 8.3 Numerical Modeling and Analysis
9. Comparison of Unretrofitted and Retrofitted Specimens
10. Future Work
11. Acknowledgments
12. Resources

👉 Experimental Retrofit of Non-Ductile Shear Walls: A Study on Failure Modes and Retrofitting Strategies

In this article, we will delve into the experimental retrofit of non-ductile shear walls, focusing on the specific case of structures built in California before the 1970s. These buildings pose a significant risk in the event of a seismic event due to their lack of compliance with current seismic standards. The primary motivation behind this project is to ensure the safety of the people inhabiting these structures.

👉 Motivation for the Project

The prevalence of pre-1970s structures in California that do not meet current seismic standards has raised concerns about their safety during large earthquake events. These buildings, particularly those with reinforced concrete shear walls, are at risk of various failure modes such as diagonal failures, flexal failures, lap splice bar failures, shear sliding failures, and out-of-plane wall buckling. The commonality of these failure modes necessitates the development of retrofit strategies to enhance the structural integrity of these non-ductile shear walls.

👉 Failure Modes of Reinforced Concrete Shear Walls

3.1 Diagonal Failures

Diagonal failures occur when diagonal cracks develop in the web region of the shear walls. These cracks start at the base of the wall and gradually increase in angle, forming a full diagonal strip from each corner of the wall.

3.2 Flexal Failures

Flexal failures are characterized by cracks that occur in the web and column regions of the shear walls. These cracks tend to localize and open up as the drift ratio increases, leading to the fracture and failure of the reinforcement.

3.3 Lap Splice Bar Failures

Lap splicing, a common practice in shear walls, can result in failures when the lap splice region is not adequately reinforced. This can compromise the overall integrity of the wall.

3.4 Shear Sliding Failures

Shear sliding of a shear wall can occur when there is insufficient lateral resistance, leading to structural instability and potential collapse.

3.5 Out-of-Plane Wall Buckling

Out-of-plane wall buckling can be a significant issue for shear walls, especially those with inadequate reinforcement ratios. The buckling can cause significant deformation and compromise the overall structural stability.

👉 Existing Building Types in California

To gain a comprehensive understanding of the retrofitting requirements, it is essential to analyze the existing building types in California. Two common types of shear walls include pylisters or barbell sections and rectangular walls with lap splices. Pylisters have larger clumps of concrete on the ends, resembling a big I-beam section. Rectangular walls, on the other hand, exhibit lap splices, making them more vulnerable to failure.

👉 Typical Material Properties

The material properties of older buildings in California are significantly lower compared to current strengths. Concrete in these structures typically has a compressive strength of just 3,000 to 4,000 pounds per square inch (PFI), while steel strength is limited to approximately 40,000 pounds per square inch (KFI).

👉 Objectives of the Research

The main objective of this research is to investigate the typical damage patterns that occur during an earthquake event for non-ductile shear walls. Additionally, the study aims to explore retrofitting strategies that are cost-effective, environmentally friendly, and capable of ensuring the safety of these structures. Numerical modeling will be employed to validate the experimental results and provide design recommendations for safe and effective retrofitting.

👉 Retrofitting Strategies

Two primary retrofitting strategies being evaluated are the use of Fiber Reinforced Polymer (FRP) and shotcrete. FRP involves anchoring fiber reinforcement into the columns and utilizing it to hold the web of the wall together, preventing the formation and propagation of cracks. Shotcrete, a technique that involves spraying a mixture of cement, sand, and aggregates onto the surface, offers additional strength and can significantly improve the structural integrity of non-ductile shear walls.

👉 Experimental Testing and Results

Several experimental tests have been conducted to evaluate the performance of retrofitted shear walls. The tests involved both control specimens and retrofitted specimens using FRP. The results showed improved ductility and a shift towards flexural failure modes in the retrofitted specimens.

👉 Comparison of Unretrofitted and Retrofitted Specimens

While the retrofitting process did not significantly increase the strength of the shear walls, it greatly enhanced their ductility. The failure modes observed in the retrofitted specimens were primarily flexural, with increased utilization of vertical reinforcement in the columns. This shift towards ductile failure modes is crucial in ensuring the safety of occupants during seismic events.

👉 Future Work

Future research efforts will focus on testing the effectiveness of shotcrete as a retrofitting strategy. The ongoing project aims to validate numerically and experimentally the benefits of shotcrete retrofitting. By combining the results with the already tested FRP retrofitting strategy, comprehensive recommendations for safe and effective retrofitting will be provided.

👉 Acknowledgments

This project is made possible through the support and sponsorship of the National Institute for Standards and Technology. We would like to express our gratitude to Simpson Strong Tie for generously donating the materials for our experimental testing. Furthermore, we extend our appreciation to the principal investigators and our advisors, Dr. Ganos and Dr. Juan Marcio Delo. Their expertise and guidance have been invaluable throughout this research.

👉 Resources

• National Institute for Standards and Technology (NIST)
• Simpson Strong Tie (www.strongtie.com)