NUMERICAL INVESTIGATION OF THE SEISMIC RESPONSE OF AN UNREINFORCED MASONRY RESIDENTIAL BUILDING HIT BY THE ZAGREB EARTHQUAKE IN 2020

Despite the moderate intensity, the series of earthquakes in Zagreb (2020) caused significant social and economic impacts and damage to the built environment. The city of Zagreb has a moderate seismic hazard, but it is highly exposed (densely populated) and the built environment is quite vulnerable (age of structures, low maintenance, illegal construction, and numerous reconstructions). The greatest damage was sustained by unreinforced masonry (URM) buildings for residential use in the historic downtown of Zagreb built in the late 19th and early 20th centuries. In addition to material deterioration, the transformations suffered by these buildings – often without being driven by anti-seismic standards - may increase their seismic vulnerability. Within this context, the main goal of the paper is to analyse the seismic response of an unreinforced masonry residential building selected to be representative of the existing historical masonry heritage in Zagreb downtown, built in the early twentieth century. The URM building has a rectangular floor plan and a 4-story elevation. Numerical investigations are carried out by using the equivalent frame method implemented in Tremuri software and by performing both nonlinear static and dynamic analyses. Since it is part of a typical residential block in the centre of Zagreb, the case study was analysed in two configurations: considering it isolated from the rest of the aggregate and sandwiched between two adjacent structural units. The results reported in this paper must be intended as a preliminary step for addressing future developments oriented to deepen the effects of interaction with adjacent buildings varying the position of the structure into the aggregate as well as those due to possible transformations and strengthening interventions.