Numerous experimental studies have been conducted on the behavior of strengthened columns, mostly with glued layers of FRP before load application, and rarely under the pre-existing load. Results confirm a good efficiency of strengthening in the compression-controlled region of the columns, as mentioned in design codes and guides such as ACI 440.2R-17. However, their behavior in the tension-controlled region is still a challenging subject. Moreover, when the eccentricity is large enough, the efficiency of the strengthening method needs more attention. This study aimed at strengthening six RC columns in the tension-controlled region under the pre-existing load condition. Furthermore, tested specimens were verified using a nonlinear finite element analysis performed in ATENA software. Moreover, some identical strengthened specimens, which were not subjected to pre-existing load during strengthening, were modeled to capture the effect of pre-existing loads. The results of the analyses were in good agreement with experimental data. Comparison of numerical results obtained for columns strengthened without pre-existing load, and under pre-existing load showed that pre-existing loading significantly reduced the efficiency of FRP jacketing. Also, assuming a linear variation of confinement pressure in the tension-controlled region of the interaction diagram results in good agreement with attained results.