Retrofit of masonry infills: local interaction with RC frames

The achievement of code-based performance criteria in existing reinforced concrete buildings is often gained through different retrofit techniques aimed at improving the seismic behavior of both structural members and non-structural elements, such as infill walls. Focusing on masonry infill walls, state-of-art strengthening techniques are often adopted to increase the in-plane capacity and to prevent out-of-plane mechanisms. As stated by several studies, the interaction between the surrounding frame and the infill panels leads to a significant effect on the dynamic behavior of the structure and often modifies its collapse mechanism. Particularly when dealing with older buildings, the brittle failure of columns with poor transverse reinforcement can be triggered by the lateral forces transferred from the infill panel. The influence of the retrofitting of the infill walls on the failure modes of buildings built prior to modern seismic provisions is investigated in this study. A numerical macro-model reproducing the lateral response of the strengthened panel has been developed. The numerical model is calibrated through experimental data and analytical formulations available in the literature. Non-linear static analyses are performed to assess the effect of the strengthening on the in-plane distribution of inertia forces and on the failure mode of the frames. The results represent a useful mean to assess the efficacy of the retrofit techniques that, despite reducing the damage to the masonry infills, can adversely affect the global performance of the RC frames in case of earthquake. Design recommendations are provided.