عنوان مقاله [English]
Reinforced concrete buildings constructed before 1970s lack seismic detailing and their longitudinal bars have been implemented in three following manners: without lap splices, with straight lap splices and with hooked lap splices. In this study, the seismic retrofitting of old-type RC columns with plain bars via near surface mounted (NSM) technique as a modern and practical method was experimentally evaluated, and also the effect of different lap splices of longitudinal bars on the behavior of columns was investigated. Six half-scale columns were tested under combined constant axial and quasi-static cyclic lateral loads, including three control specimens with different lap splices and lacking seismic detailing, and three specimens strengthened via NSM technique with steel bars. The results indicated that the retrofitting method has significantly increased the flexural strength and improved the seismic parameters, e.g. energy dissipation, hysteresis damping, and damage index. The hysteresis loops of control specimens were flag‐shaped with high pinching and low dissipated energy, due to slip of longitudinal bars. In specimens strengthened with NSM steel bars, the strength and the area of hysteresis loops increased considerably, and the pinching of hysteresis curved declined, due to the ductility of the steel materials. In strengthened specimens, buckling of steel bars happened at a length about 10 times of NSM bar diameter at drift ratio over 5%, and the wrapping was ruptured at this zone. Finally, the fracture of NSM bars happened due to low cycle fatigue. Due to the formation of struts in the core of the control specimen with hooked lap splices, severe damage was inflicted at the spliced region which decreases the axial loading capacity of these columns after an earthquake. Therefore, their seismic retrofit is a top priority. The proper wrapping of the spliced region of these columns prevents the damage, and the formation of the struts between the hooks increases the lateral strength and improves their seismic performance.