Abstract
Many existing reinforced concrete buildings designed in accordance with pre-1971 codes are generally dominated by weak column-strong beam behavior under seismic loading due to inadequate reinforcement detailing. This behavior can lead to premature failure under seismic loads from damage concentrated in the first story of the structure. This paper presents the results of an experimental investigation into the seismic response of a full-scale, two-story non-ductile reinforced concrete frame. The frame was retrofitted with a fiber-reinforced polymer jacketing system on the first story columns to mitigate seismic vulnerability. Shake weight testing was performed to investigate the dynamic performance of the retrofitted building structure in terms of the modal response, inter-story drift, and effectiveness of the fiber-reinforced polymer jacketing system. The results demonstrate that the retrofit scheme helped develop a more uniform story drift distribution, working to counter the soft-story mechanism commonly found in reinforced concrete frames designed during this period.
1. Introduction
Past seismic events (e.g. the 1989 Loma Prieta earthquake, and the 1994 Northridge earthquake) have demonstrated that many existing reinforced concrete (RC) frame buildings constructed prior to 1971 have a structural vulnerability to seismic loading. This vulnerability can be attributed to inadequate reinforcement detailing in frame columns, including: (1) large spacing of small-diameter transverse reinforcement, leading to poor concrete confinement and inadequate lateral support of longitudinal reinforcing; (2) 90 L-shaped corner hooks for rectangular column ties, resulting in loss of confinement and longitudinal reinforcement support after concrete cover spalling; and (3) inadequate lap splice lengths, causing low lateral resistance at high bending moment areas [1–9]. Inadequate detailing in combination with a low RMc/RMb ratio (where Mc and Mb are the moment capacities of columns and beams in the structure) often results in weak column-strong beam (WCSB) behavior in non-ductile RC frames.
5. Conclusions
The present work investigated the dynamic response of a twostory two-bay non-ductile RC frame retrofitted with an FRP jacketing system on columns in the first story. The effectiveness of the FRP jacketing system in improving the dynamic performance of the frame was evaluated using an experimental study. Based on this investigation, the following conclusions can be drawn:
(1) During the Phase 1 loading sequence, as the loads were increased, the natural frequencies of the retrofitted frame slightly decreased. No visible damage was observed during this stage of the loading, but it is assumed that minor damage was occurring. Additionally, the peak inter-story drift ratios of the retrofitted frame were within the Immediate Occupancy (IO) level (inter-story drift ratio 61.0% defined in FEMA 356 [54]). During the Phase 2 loading sequences (sinusoidal pulse excitation), the peak inter-story drift ratio of the first story reached the drift Life Safety (LS) level after the concrete cracking (see Fig. 10) occurred in the slab immediately adjacent to the first story column base and cover spalling. Consistent with the drift LS level, the maximum hinge rotation of the first story columns reached the rotation LS level.
