Keywords: irregular walls with openings, unreinforced masonry, historical buildings, finite element, modelling, sensitivity analysis, in-plane seismic capacity, offset irregularity.

Unreinforced masonry (URM) buildings frequently have load-bearing walls with an irregular arrangement of openings. This is the case of many existing URM buildings designed in compliance with past non-seismic codes. Previous research has shown that seismic response of irregular walls with openings to in-plane lateral loads may be strongly different from that of regular walls. Nonetheless, current seismic codes do not provide rules for an appropriate capacity modelling of these structures.

This paper deals with the in-plane seismic capacity of irregular URM walls with vertical opening offsets, namely a misalignment between upper edges of openings at the same storey. The influence of offset irregularity on seismic capacity was investigated using nonlinear finite element analyses. The finite element model was calibrated using past experimental results on a single-storey brick masonry wall specimen with two openings. A sensitivity analysis allowed the derivation of regression models for prediction of capacity variations as offset irregularity changes. It was found that seismic capacity has drastically different variations depending on the orientation of lateral loads. As far as minimum values of capacity features are considered, elastic stiffness, ultimate resisting force, and ultimate displacement strongly reduce as offset irregularity increase.

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