Seismic fragility assessment of existing masonry buildings in aggregate located in Zagreb

The historic center of Zagreb’s Lower Town presents a moderate seismic hazard but high exposure and vulnerability. This vulnerability was also demonstrated by the earthquake that struck Zagreb on March 22, 2020 (ML=5.5), which led to significant socio-economic repercussions and damage to the historic buildings. The paper analyses the residential buildings in Zagreb’s historic downtown and aims to develop tools useful for urban-scale seismic risk assessment. To this end, the seismic response of unreinforced masonry (URM) buildings in aggregate representing the city’s construction was numerically analyzed using the equivalent frame approach implemented in Tremuri software. Special attention was given to evaluating the aggregate effect by analyzing the Structural Unit (SU) both in isolation and aggregate configurations. Another aspect analyzed is the evaluation of out-of-plane (OOP) mechanisms and how they impact the development of global fragility curves. The analysis of local mechanisms was carried out using the rigid block model assumptions, with seismic input from accelerograms derived from the 3D model. For this purpose, a detailed analysis was conducted on the structure’s dynamic response variation with increasing damage and on the related floor spectra. For the reference building, it was found that the aggregate effect is beneficial for the single SU, while the OOP mechanisms lead to a considerable setback of the fragility curves due to the reduced in-plane (IP) vulnerability of the building, and thus the lack of a filtering effect by the structure.