Effects of in-plan irregularity of RC framed buildings on the out-of-plane nonlinear seismic response of masonry infills

Masonry infills (MIs) are typically found in reinforced concrete (RC) framed buildings, but they are commonly neglected in seismic design practice. As expected, the increase of lateral stiffness induced by the in-plane (IP) behaviour of MIs may emphasize or mitigate torsional effects in a building with asymmetric plan, depending on their distribution in the perimeter frames and potential damage related to the out-of-plane (OOP) failure modes. The present work is aimed at identifying the effects of lower and upper bound IP and OOP nonlinear modelling assumptions of MIs on their seismic damage. To this end, a five-storey RC framed structure, characterized by an L-shaped plan and bays of different length, is chosen as representative of a spread typology of residential buildings in Italy. A simulated design of the fixed-base structure is preliminarily carried out in line with a former Italian code, for medium-risk seismic region and typical subsoil class. Then, seismic retrofitting with elastomeric and sliding bearings is carried out, in order to attain performance levels imposed by the current Italian code in a high-risk seismic zone. Four structural models are considered: i.e. the bare structure, with nonstructural MIs; three infilled structures, with MIs arranged in order to reduce (elastic) torsional effects of the fixed-base structure along one or both the in-plan principal directions. Nonlinear dynamic analyses are carried out by employing a self-built C++ code for RC infilled framed structures. Records of recent earthquakes in central Italy are considered, matching on average the design response spectrum of acceleration for the geographical coordinates at the selected site.