Development of Backbone Curve for Reinforced Concrete Frames with Confined Masonry Walls

The response of RC frames with confined masonry (CM) walls is governed by the interaction between the surrounding RC frames and the confined masonry panels. This interaction will not only affect the structural integrity of the masonry walls but also significantly affect the failure mode of the RC frames compared to what they were originally designed for. This study investigates a simple engineering tool developed previously for infilled frames that are expressed by the relative stiffness and relative strength. To determine its validity to identify the ultimate failure modes of columns in RC frames with CM walls, the tool was calculated based on 48 laboratory specimens of single-story and single-bay CM frames tested by other researchers. Based on the obtained results, a modification to identify the ultimate failure mode of RC frames with CM walls is suggested. These experimental data were further used to develop a backbone curve of lateral response of CM frames vulnerable to fail in shear. The results of this study indicate that the proposed curve can satisfactorily represent the response of CM frames, with ranges of error in estimating the peak strength, residual strength, drift at peak strength, and drift at residual strength for all considered specimens based on proposed backbone curve of $-30\%$ to 33%, $-28\%$ to 54%, $-45\%$ to 89%, and $-35\%$ to 100%, respectively. Moreover, the proposed backbone curve is compared with a curve developed by other researchers, and it was found that the proposed backbone curve can better represent the experimental data. The maximum errors in estimating the peak strength, residual strength, drift at peak strength, and drift at residual strength for three of the specimens based on the proposed backbone curve are 11%, 16%, 67%, and 24%, respectively, while the maximum error in estimating these values based on the other researchers’ curve are 48%, 71%, 69%, and 75%.

The shear failure observed in CM frames after earthquakes encouraged researchers to investigate the reasons of this brittle type of failure. Development of an engineering tool is essential to identify when shear failure can occur based on the geometry, material properties, and reinforcement detailing, and this was previously developed for infilled RC frames. This research modified this tool to be used for CM frames based on an experimental database. It is also vital to evaluate and predict the CM frames’ behavior under lateral loads. The proposed backbone curve is effective because it allows for the assessment of existing CM frames or the design of new confined buildings taking into consideration the gravity and lateral loads to determine the strength, deformation capacity, and ultimate failure modes.