Assessment of the Additional Shear Demand induced by Masonry Infills on Reinforced Concrete Elements in presence of Seismic Actions

The influence of masonry infills in a reinforced concrete (RC) structure is often underestimated in the structural design and assessment of existing structures. Despite of a non-structural role of the infills, they can have beneficial or adverse effects on the bearing capacity of a building and thus they should be taken into account. An additional shear due to the presence of the infills can be the cause of a brittle failure of the columns. In such a framework a refined numerical micro-model was created by means of the softwares OpenSees and STKO that allowed to simulate the complicated behavior of the RC frame-infill interaction. A strategy to simulate the infills was chosen in which both bricks and mortar joints are modeled using 2D plane-stress continuum elements with a non-linear behavior. The frame-infill interaction was modeled as a contact that allowed to take into consideration the friction between the frame and the infills and at the same time to simulate the detachment of the infills from the frame. The latter was also modeled by means of 2D continuum elements and to be able to connect the concrete with fiber-section bars an embedded contact with a penalty stiffness that enforced the constraint was used. This approach of modelling the frame allowed to combine flexural and shear failure mechanisms. Eventually by comparing the results of the current model with the results of a simplified macro-model with one diagonal strut an additional shear was measured. Based on a parametric study an analytical model was proposed that allows to use a simplified macro-model without underestimating the additional shear.