## 4 Experimental Results and Analysis
### Load-Displacement Relationships and Strains  
The hysteretic behavior of each specimen under cyclic loading, along with the envelope curves for all test specimens, are presented in Figures 8 and 9, respectively. The maximum load of the NRC specimen was 322 kN in the positive direction and 314.2 kN in the negative direction when the drift ratio was 2%. As the load increased, shear cracks developed at the bottom of the column and spalling of the concrete occurred. The maximum load subsequently decreased to 311.4 kN (94% of the maximum load) during the first cycle at a 2.5% drift ratio, and the experiment was terminated as a result of shear failure at the bottom left of the column. In the case of the WCFST specimen, the load reached a maximum during the first cycle at a 2.5% drift ratio; the load was 1013.3 kN in the positive direction and 940.5 kN in the negative direction, about 3.15 times higher than that observed for the NRC specimen. The test results are summarized in Table 3. When the drift ratio was 0.1%, the load capacity of the WCFST specimen attached to the CFST frame was approximately 1.9 times that of the NRC specimen attached to a non-seismically designed frame. This disparity in load capacity became greater as the drift ratio increased, and the load capacity of the WCFST specimen was 2.8 times that of the NRC test specimen, at a drift ratio of 1.5%; it was at this point that shear cracks occurred in the columns of both specimens. These observations indicate that for the same lateral displacement, the load was greater for the specimen reinforced by the WCFST system, thereby confirming that the seismic retrofitting method using the CFST frame effectively enhances the strength of existing reinforced concrete frame structures. Figure 8: Load–displacement curves: (**a**) non-seismic reinforced control specimen (NRC) and (**b**) WCFST. Figure 9: Envelope curves. The strain values measured in the beams and columns of each test specimen during the cyclic loading test are shown in Figures 10 and 11, respectively. The strain values measured for the main rebars and stirrups at the end of the beams in the NRC and WCFST specimens were similar up to a drift ratio of 0.75%. However, from a 0.75% drift ratio until failure, the strain of the WCFST specimen was more than twice that of the NRC specimen. It was confirmed that the strain on the main rebars and stirrups increased because of the increased stress in the main rebars and stirrups, as the maximum load of the WCFST specimen was approximately three times that of the NRC specimen. The CFST attached to beams of the WCFST specimen did not yield at a drift ratio of 2.5% (where the experiment was terminated), nor did the strain increase significantly. As shown in Figure 11, the strain of the concrete at the end of the column in the NRC specimen exceeded 0.003, the ultimate compressive strain of concrete, after a drift ratio of 1% was reached. For the longitudinal and transverse reinforcement of the column, the strain for all specimens reached 0.0002, the yield strain of steel, at drift ratios of 1.5% and 2%, respectively. While the strain increased gradually after the yield of rebars in the WCFST specimens, the strain values for the longitudinal and transverse reinforcement in the NRC specimens increased rapidly during the third cycle with a 2% drift ratio. This indicates a significant development in the deformation of the longitudinal and transverse reinforcement as the concrete exceeded the ultimate compressive strain and the rebar yielded. In the NRC specimen, spalling in the concrete was observed at the bottom of the column and the cracks progressed rapidly, as shown in Table 2. The strain of the CFST at the end of the column showed a strain value less than 10% of the 0.001375 yield strain and, thus, there was little deformation of the CFST. In addition, it was determined that no deformation of the CFST occurred upon failure of the test specimen because the yield strain of the CFST was not reached at a 2.5% drift ratio (when the test was terminated).