The italian existing buildings are characterized by numerous examples of RC frames with masonry infills. Various studies and evidences from recent earthquake events showed how the presence of masonry infills may alter the overall behaviour of structures when subjected to seismic action. In general the presence of the masonry infills can produce positive or negative effects. In the first case it can increase the strength and the dissipative capacity of the structure, in the second case it can produce unexpected distributions of forces and consequent storey collapse. However, the most widespread and established practice of design does not consider the infills as structural elements, ignoring their strength and stiffness. For this reason, in the analytical models these elements are often neglected. The aim of this study is to evaluate how the presence of the infills could modify the structural response of the frames. To this purpose it has been necessary to calibrate an analytical reliable model which could account for the presence of the infills. In this study an equivalent strut model has been considered. The model for monotonic loading has then been extended to the case of cyclic loading by calibrating the degradation of strength and stiffness, the residual strength and the loading and unloading branches. This calibration has been performed by comparing the analytical results with the ones of available experimental tests on infilled frame. The calibrated model has then been used to perform several pushover and nonlinear incremental dynamic analyses considering a set of bare and infilled RC multi-storey frames. In these analyses the collapse mechanisms and the displacement profiles of the infilled and  bare frames have been compared. By examining the results of this comparison it has been possible to derive a method for estimating the displacement profile of infilled RC frames.