Numerical investigation of the displacement incompatibility between masonry infill walls and surrounding reinforced concrete frames

In the European building practice, masonry infill panels have been widely adopted as facade elements in Reinforced Concrete (RC) frames in order to provide architectural needs such as thermal and acoustic insulation. During seismic shakings, infill wall panels and the surrounding RC frame have a strong interaction, potentially leading to local brittle failures of both structural and non-structural elements or even to global collapse mechanisms (e.g., soft-story mechanism). In the past years, a significant research effort has been dedicated at the international level to better understand the seismic performance of infilled RC frame structures as well as to develop suitable and practical design/retrofit techniques to reduce the negative effects of infill-frame interaction. However, past numerical and experimental investigations mainly focused on the diagonal compression strut mechanism and associated stress path. On the other hand, a procedure to assess the local infill-frame displacement incompatibility (i.e., detachment due to the relative deformation mechanism) in terms of shape and values is still missing in the literature. Therefore, this paper investigates and discusses the seismic displacement incompatibility between infill walls and the RC frame structure as well as the key parameters affecting the infill-frame detachment. Specifically, the concept of shape functions is introduced and proposed to assess the seismic infill-frame displacement incompatibility, in line with and extending the state-of-the-art investigations on the relative deformation mechanism between seismic-resisting frames and precast flooring units. The proposed methodology can support a displacement-compatible design check of specific connection solutions, in the form of either shear keys and/or steel dowels, as part of either strengthening or decoupling seismic retrofit strategies, as well as of energy rehabilitation solutions, such as external thermal insulation systems, in order to protect these components during earthquakes.