The damage observed in past earthquakes, has shown as the presence of infills significantly influences the seismic performance of RC moment resisting frames. In fact, the presence of full infilled frames, with regular distribution, could improve the seismic response and mitigate horizontal displacements. On the other hand, an irregular distributions of infill could dramatically modify the stiffness and resistance of structures resulting in a brittle collapse mechanism related with soft story or torsional effects. In this study it has been evaluated the seismic behaviour of frames designed to bear only gravity load; a simulated design procedure has been adopted, according to code provisions and design practices in force in Italy between 1950s and 1970s. A parametric study has been performed to take into account the typical mechanical properties of masonry available in Italy. Infill panels have been modelled by means of equivalent struts. A pushover analysis has been carried out to evaluate the capacity curves and collapse mechanisms of infilled frames. The performed analysis allowed to analyse the influence of infill properties on the ductility of existing RC frames. The results emphasize as the properties of masonry infills should be adequately evaluated before doing a seismic analysis as the masonry shear strength significantly influences the global seismic behaviour of RC frames.
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