The effectiveness of seismic retrofitting of multi-storey multi-bay RC-frame buildings by converting selected bays into new walls through infilling with reinforced concrete (RC) was studied experimentally at the ELSA facility of the Joint Research Centre (JRC) in Ispra (Italy). A full-scale model was tested with the pseudo-dynamic (PsD) method and consisted of two four-storey (12m tall) three-bay (8.5m long) parallel frames linked through 0.15m slabs with the central bay (2.5m) infilled with a RC wall. The frames were designed and detailed for gravity loads only and are typical of similar frames built in Cyprus in the 1970´s. Different connection details and reinforcement percentages for the two infilled frames were used in order to study their effects in determining structural response. In order to analytically simulate the experimental response, two mathematical models were formulated differing at the level of modelling. The first model is a micro-model based on 2D finite elements while the second is a 3-D macro-model with line elements to model the bounding frame and shear wall elements to model the walls. In this paper the experimental model is described and experimental results are presented along with conclusions for the behaviour of the frame. Then, the two analytical models described above are presented along with comparison of the analytical results to the experimental ones. Finally, conclusions are drawn regarding the effectiveness of the analytical models to capture the behaviour of the proposed structural system.