Existing masonry buildings are often composed by ‘irregular’ walls with openings, namely walls having misaligned openings or different numbers of openings per storey along the height, opposed to ‘regular’ walls. The irregular layout of openings induces not only a non-uniform distribution of gravity loads among the masonry panels of the wall, but also a higher seismic vulnerability.

This paper deals with a preliminary assessment of the effects of irregularities on the non-linear behaviour of masonry walls with openings. Based on specific macro-element modelling criteria for irregular walls with openings derived from both numerical analyses and visual observations of past earthquake damages, wall irregularities have been quantitatively defined through several indices and their effects on seismic behaviour have been assessed by means of sensitivity analyses and non-linear regression analyses. Sensitivity analyses consisted in a number of non-linear incremental static analyses of two-storey masonry walls with openings and allowed to assess key seismic capacity parameters. Regression analyses let to describe each capacity parameter under varying irregularity indices. Structural analyses were carried out for both orientations of seismic actions and for different couples and layouts of openings. In particular, effects of misaligned openings in the horizontal and vertical directions were investigated.

Both sensitivity and regression analyses have highlighted that, as the magnitude of the irregularity under consideration increases, the masonry wall undergoes: (1) a slight increase in the lateral stiffness and hence in the fundamental period of vibration; (2) a dramatic reduction in the available displacement ductility, which is only somewhat compensated by a minor increase in the allowable peak ground acceleration; and (3) a significant drop in the strength reduction factor. Wall irregularities can generate undesirable damage localizations in some masonry panels, resulting in the premature collapse of the wall. For the walls analysed, major capacity variations have been found for horizontal irregularities rather than vertical irregularities.

The results of this study can provide valuable hints also on the effects of infills within existing openings, which are common interventions motivated by architectural or structural purposes (e.g., modification of stiffness/strength distribution in plan, reduction of compressive forces on some masonry piers).