This article investigates the efficiency of various retrofitting schemes using carbon fiber–reinforced polymers in improving the seismic performance of a substandard masonry-infilled reinforced concrete frame. One virtual five-story reinforced concrete frame was designed according to out-of-date Chinese codes. In total, 15 carbon fiber–reinforced polymers retrofitting schemes were adopted before earthquakes, and three sets of earthquakes representing low, medium, and high frequency contents were selected to conduct the incremental dynamic analysis. The influence of infills’ collapse due to out-of-plane effect was discussed detailedly, and then, the effectiveness of different retrofitting schemes was evaluated. It was found that the collapse of infills obviously reduced the seismic capacity and finally resulted in a soft-story failure mechanism for the reinforced concrete frame. For earthquakes with low frequency content, the enhancement efficiency of retrofitting infills or both columns and infills was increased with the increase in the number of retrofitted stories; however, for medium and high earthquake frequency contents, retrofitting infills or both columns and infills were inefficient when less than half of the structure height was retrofitted. Among the adopted 15 schemes, carbon fiber–reinforced polymers retrofit of both columns and infills along the full building height was found to be the most efficient. Improper selection of a retrofitting scheme could lead to the change of soft-story location. The inter-story drift ratio capacity of effectively retrofitted frames can meet the requirements of current seismic code.

Introduction

The masonry infills are frequently used as interior partitions and exterior walls in buildings. They are usually considered as non-structural elements, and the interaction with the bounding frame is often ignored in design or seismic performance evaluation of reinforced concrete (RC) frame structures. Such an assumption is not always a conservative approach although the presence of masonry infills can increase the initial global stiffness and lateral load-carrying capacity of RC frames. It is due to the fact that the stiffer the building is, usually, the higher seismic lateral loads it attracts. Extensive collapses of masonry-infilled RC frames in the past earthquakes (e.g. San Fernando 1971, Northridge 1994, Kobe 1995, Jabalpur 1997, Wenchuan 2008, L’Aquila 2009, Port-au-Prince 2010, Lorca 2011) have revealed that the masonry infills have a negative influence on the seismic performance of infilled frames, such as the soft-story failure mechanism caused by vertical discontinuity of masonry infills (Dolsek and Fajfar, 2001; Verderame et al., 2011), torsional failures induced by the asymmetry of infills in the plan (Correnza et al., 1994; De Stefano et al., 1998), and short-column effects due to the openings in infill walls (Bikce, 2011; Cagatay et al., 2010). Even for the RC frames with regularity in plan and elevation, the collapse of masonry walls may also lead to vertical or plan irregularity and further result in the unexpected soft-story or torsional failure mechanisms for the whole structure during earthquakes. In recent years, although numerous studies have been conducted to investigate the effect of masonry infills as structural elements on the overall seismic responses of RC frames, most of these investigations concentrated on the new buildings. However, a large number of existing masonry-infilled RC frames, which were designed and built according to out-of-date codes, are still in service in potential earthquake regions across the world. These RC frames are commonly referred to as substandard or nonductile frames (Baran and Tankut, 2011; Ozcelik et al., 2011; Wang et al., 2016; Zou et al., 2007). The masonry infills of these frames are generally poorly connected to the bounding frames and thus face at high risk of collapse under the future strong earthquakes. Following major earthquake events, unsatisfactory performance (i.e. collapse) of masonry infills in these frames has been repeatedly reported (Braga et al., 2011). To improve the seismic safety of RC structures, there is an urgent need for the seismic retrofit of existing substandard infilled RC frames before earthquakes, which has become a key issue for the engineers around the world.