## 2 Seismic Retrofitting of Reinforced RC Structures Using CFRP Sheets  
Recent earthquakes have underscored the vulnerability of existing structures to seismic loads, emphasizing the urgent need to address the structural integrity and seismic resilience in older buildings. These events have highlighted the shortcomings of many existing structures, particularly those built before modern seismic codes and standards were established. Consequently, there has been a growing focus on seismic retrofitting methods to enhance the resistance and performance of reinforced concrete structures against severe ground motion. One widely employed method in seismic retrofitting is the use of carbon fiber-reinforced polymer (CFRP) sheets to strengthen beams and columns. Civil engineers frequently turn to this technique due to its effectiveness in enhancing the seismic performance of structures. However, the critical challenge lies in the proper design of the retrofitting scheme. Achieving the desired level of performance while maintaining the structural integrity and stability of the existing building is of paramount importance. A well-executed retrofitting plan not only bolsters the structure's ability to withstand seismic forces but also ensures that the retrofit elements integrate seamlessly with the original construction, minimizing the risk of unforeseen issues. In the ongoing effort to mitigate earthquake-related risks, special attention to retrofit design and execution remains a vital aspect of safeguarding vulnerable structures and, ultimately, the safety of the people who inhabit them. The interest in the use of CFRP materials in the retrofitting of defective RC frame beam and column connections is growing remarkably, as the methods have advantages, for example:  
(i) Improved seismic performance: Beam-column connections are critical areas in a structure, and defects in these areas can severely deteriorate seismic performance of buildings. CFRP materials can be effectively used to strengthen these connections to increase the ductility, which led to better the withstanding of earthquake forces. This increase in ductility helps to prevent easy failures and ensures the structure's resilience in the event of an earthquake by reducing catastrophic damage. (ii) Lightweight and corrosion resistant: CFRP composites are lightweight as compared to the conventional steel jacketing method. This advantage makes them particularly suitable for retrofitting and strengthening work as they add less weight to the existing structure. Furthermore, CFRP materials do not corrode, which is a significant advantage over steel reinforcement. Steel corrosion can weaken connections over time, while CFRP retains its strength and integrity, providing long-term durability and reliability. (iii) Ease of installation: CFRP materials are relatively easy to install and can be used for retrofitting a structure to address specific defects in beam-column connections. This ease of installation can reduce maintenance work during the renovation. (iv) Tailored retrofitting solution: CFRP retrofitting can be conducted according to the specific configuration of structural elements. Engineers can target specific defects and weaknesses in a given RC frame by designing a CFRP reinforcement system. This tailored approach ensures repeatability. Therefore, these advantages make CFRP a valuable and versatile tool in enhancing the safety and longevity of existing structures, particularly in earthquake-prone regions. In this study also, CFRP sheets have been selected for retrofitting the joints among all other fibers. CFRP is preferred because it possesses higher tensile strength and stiffness compared to other types of fibers thanks to the exceptional strength of carbon fibers. Additionally, it is lighter, more corrosion-resistant, and better equipped to withstand high temperatures when compared to other fiber types. Therefore, CFRP materials are better suited for retrofitting RC joints, given their critical function.