Stick model for as-built and retrofitted infilled RC frames

The Stick model is a MDOF system composed of masses lumped at the story level, connected in series by nonlinear shear link elements suitably calibrated to simulate the interstory inelastic behavior. The model has been demonstrated to predict with sufficient accuracy the dynamic response of existing masonry-infilled RC structures in terms of damage concentration and evolution under specific ground motion records. Previous works proposed interstory backbones calibrated on the response of refined MDOF finite element models representative of gravity-load designed buildings. In this work, with the aim of extending the applicability of the Stick model to different building typologies, a simplified procedure is proposed to generate interstory backbones for the Stick model simulating the behavior of existing masonry-infilled RC structures in as-built and retrofitted configurations. The procedure allows generating Stick models accounting for different levels of design (gravity load, obsolete seismic codes), age of construction, number of storeys and in-plan shape and dimensions. Relevant features characterizing existing buildings are accounted for, explicitly simulating typical failure modes of non-conforming RC elements, brittle failures induced by local frame-infill interaction phenomena, and introducing typical building-level collapse modes for existing buildings. The proposed procedure is extended to account for the possible implementation of retrofit intervention at the building level to both increase the structural capacity and the stiffness of the building. The proposed procedure can be employed to generate typological Stick models to be adopted in large-scale loss assessment studies, also evidencing the beneficial effect of possible retrofit interventions.