Vulnerability Assessment: Comparison of Empirical and Analytical Approach – A Case Study in Zagreb, Croatia

In March 2020, the capital of Croatia was hit by two earthquakes of moderate magnitudes. The earthquakes caused high economic losses, due to damage that occurred in the city centre. The city centre is mostly made up of historical masonry structures that have proven to be extremely vulnerable in the case of an earthquake. The paper represents two approaches to estimating vulnerability: an empirical and an analytical approach, respectively. The case studies building aggregates are in the city centre, Lower Town. The application of the empirical approach requires data about previous earthquakes and the damage that occurred. Firstly, the macroseismic approach, which is an index-based method, is being used. Structures were categorized into typological classes. The results were calibrated according to data from the earthquake that occurred in 2020 and are presented in terms of vulnerability and fragility curves as well. Next, the analytical approach was considered, by performing a non-linear static analysis on the selected structural unit as a representative of the predominant structural typology present in the study area. The selected structural unit was modelled in software 3Muri, using the macro-element approach. The impact of structural interaction was considered by adopting a simplified numerical procedure. Moreover, fragility curves were derived from pushover curves. The main aim of the research was to compare the empirical and analytical approaches to validate the empirical approach. While the analytical approach is more precise, due to more input information, the macroseismic method is simpler to use and it provides results on a wider scale which is useful when developing risk mitigation strategies.