In this study, a seismic strengthening method by internally attaching steel frame was proposed for strengthening medium-to-low-rise reinforced concrete (RC) buildings. The main technique proposed in this study was a welding method for connecting between existing R/C frame and internal steel frame strengthening element. The connection technique was developed considering the structural integral behavior between existing frame and strengthening member. The method is one of the strength design approach by retrofit which can easily increase the ultimate lateral-load capacity of R/C buildings controlled by shear. The cycling load test, designed using an existing school building in Korea, was carried out in order to verify the seismic retrofitting effects of the proposed method in terms of the maximum load carrying capacity. The nonlinear static analysis was also conducted to establish a hysteresis model of the proposed strengthening method, and to investigate the consistency between experiment and analysis results. The results revealed that the proposed internal strengthening method installed in R/C frame enhanced conspicuously the strength capacities, and the test results can reasonably compared to those of the non-linear static analysis.

The friction damper can be used for improving the seismic resistance of existing buildings. The damper is often installed in bracing members. The energy dissipation capacity of the damping systems depends on the type of the structure, the configuration of the bracing members, and the property of dampers. In Korea, there are numerous low-to mid-rise reinforced concrete moment frames that were constructed considering only gravity loads. Those frames may be vulnerable for future earthquakes. To resolve the problem, this study developed a toggle bracing system with a high density friction damper. To investigate the improvement of reinforced concrete frames after retrofit using the developed damped system, experimental tests were conducted on frame specimens with and without the damped system. The results showed that the maximum strength, initial stiffness and energy dissipation capacity of the framed with the damped system were much larger than those of the frame without the damped system.