Modelling the out-of-plane behaviour of URM infills and the in-plane/out-of-plane interaction effects

Unreinforced masonry infills are non-structural elements subjected, during seismic action, to in-plane and out-of-plane displacement and acceleration demands. In-plane and out-of- plane seismic responses of such elements are not independent as in-plane damaging reduces out-of-plane capacity and vice-versa. This phenomenon is called in-plane/out-of-plane interaction. In this paper, a macro-modelling strategy aimed at accounting for both in-plane both out-of- plane behaviour of unreinforced masonry infill walls is presented. Out-of-plane strength reduction due to in-plane damaging is modelled. Out-of-plane stiffness reduction due to in-plane displacement demand and vice-versa is also considered, in order to correctly estimate acceleration demand during seismic motion. A routine aimed at removing infills from the structural model during non-linear time-history analyses at the attainment of the in-plane or of the out-of- plane collapse condition is defined. The proposed modelling strategy has been implemented in OpenSees in order to allow users to model the in-plane and out-of-plane behaviour of infills, as well as their degrading rules, adopting any material model. The effect of taking into account or neglecting in-plane/out-of-plane interaction phenomena on the results of non-linear time-history analyses by applying the proposed model is shown through the seismic assessment of an example RC frame in a non-linear dynamic framework.