engleski

Procjena potresne otpornosti tipične zgrade od neomeđenog ziđa u centru Zagreba

Seismic performance assessment of a masonry building in Zagreb, the capital of Republic of Croatia, is presented in this paper. The building is a representative example of a typical unreinforced masonry (URM) building in the Zagreb historic center. The building was built in 1920 what was before the first technical regulations considering seismic action were adopted in the country. Thus, it belongs to a typology of buildings without the necessary seismic resistance. In March 2020, a moderate ML5.5 earthquake struck Zagreb in which the analyzed building suffered severe damage. The building survey was performed on the day of the earthquake and the building was categorized as temporarily unusable. Severe damage to the partition walls and slight to moderate damage to the load-bearing walls was observed, and there was a partial collapse of gable walls due to formation of out-of-plane mechanisms. Assessing the seismic behavior of existing unreinforced masonry buildings is a particular challenge for researchers but also for the engineers who ultimately carry out retrofit of URM buildings. In practice, there is a ubiquitous discu ssion about which numerical methods are applicable and reliable enough to perform seismic performance assessment of existing buildings. Often due to the very limited resources and available time, seismic assessment based on numerical models for ordinary buildings that have a basic value of importance factor, is limited to a use of linear methods and nonlinear static procedures. For the building presented in the paper, firstly the out-of-plane wall failure mechanisms are analyzed in order to determine the necessary assumptions for the analysis of the global building response. This form of the wall failure can significantly endanger the overall vertical load-bearing capacity and stability of the structure. Further, it is important to highlight that very few countries have technical regulations that define procedures for the assessment of this form of structural failures. Out-of-plane mechanisms have been identified for this building, and the results are given in parametric form that considers different wall height and thickness. Moreover, for walls that are connected to transversal walls the effect of friction is also included. Based on this analysis, necessary assumptions for the numerical calculation of the global model are presented. A global structural response is determined firstly by a linear analysis carried out using the response spectrum which is a common method for design of a new structure. In this model, the assumption of the linear elastic behavior of the structure with cracked cross sections is used. Next, the nonlinear numerical computation using pushover method is performed. Material nonlinearity is taken into account, while geometric nonlinearity is not considered due to the high stiffness of the building and small displacements before failure. Finally, a discussion about discrepancies that are present in the parameters of numerical model is presented, emphasizing the unreliability of the model, which cannot be fully eliminated, but should be accounted for when interpreting the results, and drawing the conclusions that are based on numerical models.

unreinforced masonry, out-of-plane mechanisms, nonlinear modeling, pushover method

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