Nonlinear dynamic assessment of the out-of-plane response and behaviour factor of unreinforced masonry infills in reinforced concrete buildings

Current building codes address the issue of the seismic safety check of non-structural elements, such as unreinforced masonry (URM) infill walls. The force-based safety assessment of these elements includes the verification with respect to out-of-plane (OOP) seismic accelerations. To this aim, on one hand different force-demand models, based on simplified floor spectra, are proposed by codes; on the other hand, OOP strength-capacity models are not always provided; in addition, the capacity models proposed by codes do not always account for the OOP strength reduction due to in-plane (IP) damage, i.e., for the so-called IP/OOP interaction. Moreover, within such a force-based framework, current codes also provide behaviour/response modification factors with different values ranging from the unit to more than two. However, the source and reliability of these values have never been checked. In this study, non-linear time-history incremental dynamic analyses are performed on reinforced concrete buildings designed according to Eurocodes and different for design seismic acceleration and number of storeys. The buildings are infilled by three different infill layouts. For infill walls, a recently-proposed modelling strategy accounting for their OOP response and collapse, respectively affected and promoted by the IP/OOP interaction, is implemented. The results of more than thirty thousand dynamic analyses allow the evaluation of the seismic acceleration capacity of infills with respect to the OOP collapse and of the parameters influencing it. In addition, a robust and reliable evaluation of the OOP behaviour factor is performed. Such a value is compared with current code provisions and proposed for a potential application in the seismic safety check of URM infills within a code-based linear elastic approach.