An incremental-iterative procedure is presented for approximate assessment of the response of unreinforced masonry buildings to seismic loads. Working within the framework of the shear-type structural behavior, we propose a constitutive model for the resisting elements able to account for both the variation in lateral stiffness induced by cracking, and their sort of failure (i.e. ductile or brittle). Application to a practical case demonstrates how, when modeling walls with generic windowing, the manner in which they are subdivided into smaller resisting elements strongly influences the predicted structural response and the manner of failure of each wall.