Overturning of unreinforced masonry walls – predominant failure mechanism in historic masonry buildings

Masonry buildings, as the fundamental fabric of historic urban centers in Croatian cities, also represent the main source of damage during earthquakes. Analysis of the local failure mechanisms that caused this damage identifies in-plane and out-of-plane failure mechanisms. Out-of-plane mechanisms, besides significantly contributing to the damage caused by the December 29, 2020 earthquake in Petrinja, were rarely the focus of seismic performance of masonry buildings prior to the new generation of the HRN EN 1998 standard. The primary aim of this study is to demonstrate that the activation of such mechanisms poses a direct threat to human life and to emphasize the importance of developing skills for recognizing early signs of their formation and identifying structures prone to their activation. This is particularly relevant for field inspections following earthquakes, allowing timely assessment of the structural vulnerability of buildings and reducing risks to occupants and professionals. This study presents a categorization of out-of-plane failure mechanisms based on a methodology developed within this work for the historic core of Petrinja. The categorization is based on the geometric characteristics of documented mechanisms, their position within the building, and other relevant features, with the distribution of categories illustrated through pie charts and tables. Furthermore, a formulation of the out-of-plane wall behavior model is presented, first as a mathematical framework and then as a worked example in accordance with the guidelines of both the current and the new generation of the HRN EN 1998 standards.