###### Abstract  
This study proposes a concrete-filled tube composite strengthening system (CCSS), which is a new concept that can improve and supplement the vulnerability of existing CFT seismic strengthening methods. The CCSS seismic reinforcement method is easy to construct and integrates with the existing frame and reinforcement. It is one of the seismic strengthening methods that allows the simple calculation of the required amount of seismic reinforcement and can efficiently increase the strength when applied to existing reinforced concrete (R/C) buildings with non-seismic details dominated by shear failure. To examine the seismic safety of the proposed CCSS, two framework specimens were prepared based on an existing R/C building with non-seismic details. A pseudo-dynamic test was conducted on the unreinforced framework specimen and the framework specimen reinforced with CCSS to verify the seismic strengthening effect of applying CCSS to the existing R/C framework, i.e., in terms of the load-displacement characteristics and seismic response control capability. Based on the pseudo-dynamic test results, restoration of the force characteristics was proposed for the nonlinear dynamic analysis of the building reinforced with CCSS. Nonlinear dynamic analysis was conducted based on the proposed restoration of the force characteristics, and the results were compared with the pseudo-dynamic test results. Finally, for the commercialization of CCSS, nonlinear dynamic analysis was conducted on the existing whole R/C building with non-seismic details that was reinforced with CCSS. The seismic strengthening effect was then verified by examining the seismic response load, displacement characteristics, and the degree of seismic damage to the members based on the ductility ratio before and after seismic strengthening. The study results show that under a design basis earthquake with a magnitude of 200 cm/s2, the unreinforced R/C building exhibits shear failure, and light seismic damage is expected on the CCSS-reinforced building. e 20 October 20231 December 202  
Gyeongju earthquake, the scale of the seismic damage to buildings was small except for the seismic damage to non-structural members, including the columns of some buildings (e.g., schools and apartments) located near the epicenter. However, the 2017 Pohang earthquake caused serious seismic damage to school buildings and new pilot structures without seismic reinforcement, as shown in Figure 1[1]. Among them, the shear failure of reinforced concrete (R/C) columns with insufficient shear reinforcement is currently an important topic for seismic safety design in Korea. Therefore, owing to the increasing magnitude and frequency of earthquakes worldwide, it is important to develop economical and effective seismic strengthening methods that can ensure the seismic safety of buildings vulnerable to earthquakes, particularly R/C buildings without seismic design, which are susceptible to shear failure in the columns. This can reduce property damage and human casualties caused by the destruction of buildings during a large earthquake. In addition, seismic strengthening should be performed using efficient and economical methods. Meanwhile, representative seismic strengthening methods for existing R/C buildings without seismic reinforcement include strength reinforcement methods (the R/C wall installation method, section enlargement method, and steel brace method), deformation capacity improvement methods (the steel-plate reinforcement method, wire-mesh insertion reinforcement method, and fiber-reinforced polymer (FRP) reinforcement method), methods for improving the energy absorption capacity using seismic damping systems, and seismic isolation methods [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]. According to [11; 12], shear failure is highly likely to occur in most low- and mid-rise R/C buildings without seismic design because the interval of the stirrups in the columns is approximately 30 cm or more.