Equivalent frame idealization of walls with irregular openings in masonry buildings

Within the framework of modelling of unreinforced masonry structures, the equivalent frame (EF) approach is widely used for the seismic assessment. However, the definition of modelling rules for the EF idealization of walls is not always straightforward. Such rules are typically applied a priori and they can strongly affect the seismic response. This paper aims to investigate the reliability of four existing rules for the a priori identification of the geometry of the piers, i.e. the elements in charge of resisting the horizontal and vertical loads. This constitutes the first main issue within the EF idealization of walls. Continuum finite element (FE)-based numerical solutions are considered as reference to assess the reliability of EF approach for a number of irregular masonry walls case studies. The investigated irregularity deals with the presence of openings with different height at the same storey or the presence of small openings. The critical comparison between FE and EF solutions is made in terms of (i) pushover curves, (ii) damage patterns, (iii) generalized forces, and (iv) drift values at scale of single elements. The results herein achieved allow to provide practical recommendations for the EF walls idealization. In particular, some existing rules for the a priori identification of the geometry of piers already appeared to work properly, while specific precautions appeared to be needed for other rules or in specific cases, which have been identified. More specifically, the rules that may lead to very squat piers appeared to be particularly problematic since may produce a significant underestimation of the ultimate displacement capacity at least when compared to the one estimated by the FE model, assumed as reference solution in the paper. Finally, in the case of small openings, results suggest how the most reliable solution consists of neglecting them in the EF idealization process. To support analysts in this choice, some preliminary indications on the dimensions that identify "small openings " are also provided in the paper.