Nonlinear modeling of the seismic response of masonry structures: critical review and open issues towards engineering practice

This paper provides a comphrensive review of the critical aspects of nonlinear modeling for evaluating the seismic response of masonry structures, emphasizing the issues relevant to engineering practice. Currently, the specialized technical community shares the opinion that, for a performance-based approach, numerical models are the only tools sufficiently effective to support the seismic assessment of existing buildings. However, their potential often falls short when attempting to accurately describe the behavior of masonry structures. In fact, these structures feature highly complex architectural configurations, different masonry types, and various structural solutions, meaning that extra care is required in numerical modeling. This is especially true when the modelers do not have a solid background in the software chosen and may not be practiced using the vast variety of options offered by the software houses. They are often unaware of the consequences that questionable modeling choices may have on the results obtained by the models. These extremely complex topics are treated in the paper from an engineering practice perspective, providing an in-depth overview of the challenging issues related to the use of different modeling strategies. The paper covers strategies ranging from the Equivalent Frame approach (widely used in common engineering practice) to more refined techniques like 2D and 3D Finite Element procedures based on continuous, discrete, and micro-mechanical approaches. Critical aspects in the modeling of both in- and out-of-plane responses of masonry, as well as the critical issues in wall-to-wall connections and diaphragm roles are investigated. All the examined issues are clarified through numerical examples highlighting also how a consistent and integrated use of different procedures may be beneficial. Finally, some of most relevant challenging issues concerning the use of numerical models in seismic assessment with the nonlinear static approach are presented and discussed.