Abstract
The Zagreb 2020 earthquake has damaged many historical buildings within the old city centre despite it’s medium intensity. Damaged buildings were mostly typical unreinforced masonry buildings (URM) in aggregates built in the first half of the 20th century when seismic codes were not sufficiently developed and they are considered high-risk in case of an earthquake. A risk-based assessment of such a typical masonry building in the urban area of Zagreb is presented and it’s seismic performance evaluated. The nonlinear model of a four-storey building using ETABS is described and incremental dynamic analysis (IDA) is performed using 13 ground motion records selected according to local seismotectonic conditions. 13 IDA curves are generated up to collapse with a total of 96 performance points. Damage states are assigned by direct visual inspection of the 3D model after each IDA run using EMS-98 damage classification instead of EDP parameter thresholds. Fragility curves are derived for slight, moderate, extensive and complete damage states using IM-based approach. The vulnerability curve is derived using mean damage ratio (MDR) values according to the latest national seismic risk assessment for URM buildings in Zagreb. Results show that this type of buildings reaches slight damage state for return period TR=95 years, exceeds moderate damage for TR=225 years and reaches extensive damage and even collapse for TR=475 years. This building type is therefore very vulnerable to code-based hazard and should be retrofitted in order to increase it’s seismic performance.
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