Influence of masonry infill on the collapse performance of RC frame buildings

Unreinforced masonry-infilled reinforced concrete (RC) frames are widely seen in many parts of the world. The lateral force-displacement response of the infilled RC frames is rather complex due to multiplicity of possible brittle failure modes for the masonry panel and the interaction between frame and infills, which explains the absence of comprehensive guidelines from most design standards across the world. Most design codes treat masonry infills in the RC frame structures as non-structural elements. Some codes do consider the infills as structural elements, but do not recommend accounting for the strength and stiffness characteristics of masonry infill at the design stage. This paper presents a study on the influence of infill on the performance of infilled frames during extreme earthquakes. A total of 12 frames are considered that differ in seismic zone according to Indian earthquake code (III or V), number of stories (three or nine), presence of infill (bare frames or uniformly infilled frames), and masonry prism strength (3 MPa or 9 MPa). Bare frames are designed first considering the dead weight of masonry. Infills are introduced in the frames thus designed. A recently proposed macro model is used to model the frames. The model is validated against reported experimental studies on single-bay-multi-story infilled RC frames. Pushover analyses on the frames suggest that infills contribute to significant increase in lateral strength and stiffness of the frames, and that the ground story columns in an infilled frame attract shear forces much greater than respective design shear forces. Collapse margin ratio (CMR) was calculated in accordance with the provisions of FEMA P695. This ratio indicates the factor by which the maximum considered earthquake (MCE) spectrum would be increased to achieve a 50% probability of collapse. Increase in masonry prism strength from 3 MPa to 9 MPa led to an increase in the CMR by 60 – 190%. The CMR was considerably lower for frames in seismic zone V (higher seismic zone) compared to zone III, and for nine-story frames compared to three-story frames. The reduction in the CMR for a higher seismic zone was more pronounced for a greater masonry prism strength. Overall, presence of a “strong” infill was found beneficial for collapse performance of infilled RC frames, but it also meant that columns would attract much greater forces compared to their design forces making them vulnerable to shear failure. Noting that strength of infill is not accounted for at the stage of design of frame buildings, masonry can induce great amount of uncertainty in the performance of infilled RC frames and its individual components.