On the prediction of shear brittle collapse mechanisms due to the infill-frame interaction in RC buildings under pushover analysis

A large number of research studies deal with the modeling and analysis of infilled reinforced concrete (RC) buildings under seismic actions, at the aim to understand the actual contribution given by masonry infills to the overall seismic resistance of a building. In this paper this aspect is investigated in the framework of pushover analyses, describing the theoretical and computational choices related to the involved parameters. Differently from the approaches available in literature and standards, the "double-strut model" is adopted to simulate the infill behavior, according to which an infill panel is represented by two equivalent non-parallel struts; the peculiarity is that the positions of the extremities of the two struts coincide with the points of application of the stress resultants on each side of the panel. The results show that, by adopting the double-strut model, it is possible to capture dangerous local shear failures which are usually neglected in pushover analysis and which can compromise the safety of the overall structure. By including in the analysis shear plastic hinges together with bending ones, it is evident how the additional shear forces, arising at the extremities of beams and columns, can substantially change the collapse mechanism of a structure under seismic action. The main features of the double-strut model are its low computational cost together with its accuracy, which make it particularly suitable for applications in the engineering practice. In fact it could be easily implemented in commercial calculation codes, representing a practical predictive tool able to enhance the safety level of infilled RC buildings.