SEISMIC PERFORMANCE OF MASONRY INFILLED R/C FRAMES

It is well known that masonry infills, although non-engineered and considered as non-structural, may provide most of the earthquake resistance and prevent collapse of relatively flexible and weak RC structures. The objective of this study is to investigate the effect of masonry infills on the performance of reinforced concrete frames subjected to earthquake ground motions. To this purpose an equivalent discrete shear-type model with and without infills was used for the evaluation of elastic and inelastic response of multi-story frame structures. The masonry-infilled modeled by means of equivalent strut elements, which can only carry compressive loads, characterized by an idealized degrading hysteretic behavior. The adopted mathematical models was validated by comparing numerical and test results. To investigate the influence of the mechanical characteristics of the infills, three different idealized type of masonry infills were considered, defined as weak, intermediate and strong. The performance of a large number of different reinforced concrete two bay-frames, bare and infilled, subjected to ten ground motion was investigated. The wide range of natural periods taken into account allowed to establish response spectra for several significant parameters characterizing the behavior of bare and infilled frames. The results of the investigation suggest that the global non-linear seismic response of reinforced concrete frames with masonry infill can be simulated by a relatively simple mathematical model, which combines a shear-type model with equivalent strut elements representing the infill walls, provided that the infill does not fail out of plane. Moreover, infills, if present in all storeys, give a significant contribution to the energy dissipation capacity, reducing the dissipation energy demands in frame elements and decreasing significantly the maximum displacements. Therefore the contribution of masonry is of great importance, even though strongly depending on the characteristics of the ground motion, especially for non-seismic frames, which have a lower capacity of dissipating energy than the seismic ones.