###### Abstract
Moreover, because the horizontal load-carrying capacity is also insufficient, the use of a single seismic strengthening method to enhance the ductility is inefficient. Therefore, the strength enhancement method is more efficient and economical for securing the seismic safety of buildings. Currently, the major seismic strengthening methods include adding infill shear walls in the framework, installing various types of steel braces (e.g., V-type, K-type, and X-type) inside the framework, inserting steel panels and precast concrete panels in the framework, and employing bonding methods inside the framework (e.g., the section enlargement method). Although internal-bonding seismic strengthening methods are effective for increasing the horizontal strength of buildings [2; 3; 4], they increase the weight of buildings. In particular, buildings with weak foundations, such as R/C buildings with non-seismic features, may require foundation reinforcement owing to the weight increase. It is also difficult to prepare a workspace for seismic strengthening owing to the limited available space. For the seismic wall installation and steel brace methods, which are widely used methods, additional transport locations and internal work space for the reinforcement are required. In this case, it is disadvantageous because the space available for reinforcement construction is reduced. In addition, the available space is limited and the efficiency is  
Figure 1: Shear damage on R/C columns after the 2017 Pohang Earthquake [1]. lower because it is difficult to secure space during strengthening work, and the seismic strengthening construction period is highly likely to be elongated for maintaining the integrity of the existing framework and internal joints and for securing the demand for a high construction accuracy [4]. To address these problems, it is necessary to develop construction methods that are suitable for investigating the seismic structural characteristics (e.g., shear failure type and low ultimate strength) of existing low- and mid-rise R/C buildings without seismic reinforcement and that can maintain the integrity of the existing structure and seismic reinforcement. In addition, new internal-bonding seismic strengthening methods that enable efficient space utilization should also be developed. A concrete-filled tube composite strengthening system (CCSS), which is a new concept that can improve and supplement the vulnerability of existing seismic strengthening methods, is proposed in this study. The CCSS method has excellent joint constructability between the existing framework and reinforcement. It is one of the seismic strengthening methods that can allow the simple calculation of the required seismic reinforcement volume and can efficiently increase the strength when applied to the existing R/C buildings with non-seismic features dominated by shear failure. As shown in Figure 2, to examine the seismic safety of the proposed CCSS, two two-story framework specimens were prepared based on an existing R/C building with non-seismic features. A pseudo-dynamic test was then performed on the unreinforced two-story framework specimen and the two-story 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 load-displacement characteristics and seismic response control capability. The force characteristics were restored based on the pseudo-dynamic test results to allow the nonlinear dynamic analysis of the building reinforced with CCSS (framework specimen) and the restoration force, 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 R/C building with non-seismic features 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.