Irregular Opening Layouts in Unreinforced Masonry Walls: Equivalent Frame and Finite Element Simulations

The equivalent-frame method, compared to other modeling techniques, has advantages in terms of readability of results and comptuational efficiency. Since it is explicitly endorsed by building codes, it is widely used in the professional practice. However, in presence of irregularities in the layout of openings, a common circumstance in existing structures, it is difficult to univocally identify the equivalent frame and the reliability of the results obtained for the most irregular cases is doubtful. Here, the issue of irregularity is treated by comparing equivalent-frame results with finite-element ones, which are assumed to better represent the actual behavior of irregular walls. An equivalent-frame solver is developed, by using more refined failure criteria for the masonry piers and strength criterion for the spandrels than the oversimplified ones suggested by building codes. Automated procedures are used to perform a large number of analyses, in which equivalent-frame results and finite-element ones are compared. The difference in terms of seismic vulnerability between the two models is correlated with a measure of the degree of irregularity of the walls. A geometric confidence factor, increasing with the degree of irregularity, is prosed to account for the epistemic uncertainties in the equivalent-frame modelling.