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This paper presents first a valid, fully non-linear 2-D numerical model that can capture realistically the in-plane hysteretic behaviour of reinforced concrete (R/C) frames with masonry infills when they are subjected to combined vertical and cyclic horizontal loads in order to predict their post-elastic earthquake bahaviour. The effectiveness of this simulation was validated by comparing the numerically predicted behaviour with results from a series of pseudo-dynamic tests whereby a number of 1:3 scale, one-bay, one-story R/C frame specimens, including relatively weak masonry infills, were subjected to combined vertical and cyclic horizontal seismic-type loads. The role of the interface between the masonry infills and the surrounding concrete frame was also included in this simulation. Next, this paper deals with the applicability of this successful non-linear masonry-infill concrete-frame numerical simulation to predict realistically the seismic behaviour of prototype multi-story R/C frame structural formations with masonry infills. The major obstacle here is the computational time and memory requirements needed for the completion of such a numerical analysis including all the nonlinear mechanisms which were employed in the preceding simulation of the single-story one-bay R/C frame with masonry infills. In order to overcome this obstacle, use was made of an equivalent post-elastic "pushover" type of analysis that draws information on the stiffness and strength variation from one-bay, one-story R/C masonry infilled unit frames that compose a given multistory structural formation. In doing so, the fully non-linear numerical simulation of the single-story units that compose this structural formation, presented in the first part, is utilized.

Observations from past strong earthquake activity have shown that masonry infill panels can be damaged to a substantial degree.Under certain circumstances this interaction of masonry infills with the surrounding R/C frame during the seismic response may result in either beneficial effects, by increasing the stiffness, strength and seismic energy absorption of the structure, or in adverse consequences (damage) to the main R/C structural elements. Consequently, there is need first to understand this behavior resulting from the masonry infill to R/C frame interaction and next to have reliable tools to predict it in order to prohibit adverse consequences. These tools can then be utilized in the retrofitting of R/C multistorey structures designed and constructed according to old earthquake resistant design provisions. This paper presents first a valid, fully non-linear 2-D numerical model that can capture realistically the in-plane hysteretic behaviour of reinforced concrete (R/C) frames with masonry infills when they are subjected to combined vertical and cyclic horizontal load. The effectiveness of this simulation was validated by comparing the numerically predicted behaviour with results from an experimental sequence whereby a number of 1:3 scale, onebay, one-storey R/C frame specimens, including weak and strengthened masonry infills, were subjected to combined vertical and cyclic horizontal seismic-type loads. Next, this paper deals with the applicability of this successful non-linear masonry-infill concrete-frame numerical simulation to predict realistically the seismic behaviour of a prototype multi-storey R/C frame structural formations with masonry infills. Further more the proposed numerical simulation of the same multi-storey R/C frame structural formation was utilized in an application of a retrofitting scheme. In order to overcome the obstace of computational time and computer memory requirements, use was made of an equivalent post-elastic "pushover" type of analysis that draws information on the stiffness and strength variation from one-bay, onestorey R/C masonry infilled unit frames that compose a given multistorey structural formation.