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The seismic Performance-Based assessment of existing masonry buildings requires the use of nonlinear models, in order to check the attainment of ultimate limit states. Incremental Dynamic Analysis represents the most accurate method but very few models are available which are able to describe the stiffness and strength degradation, which are typical of masonry buildings, as well as the hysteretic behaviour of piers and spandrels under cyclic actions. At engineering practice level, the Displacement-Based approach is widely adopted, through the use of nonlinear static analysis. However, the application in the case of irregular URM buildings with flexible horizontal diaphragms represents an open issue, due to the various difficulties, for example in the selection of proper load pattern or the definition of performance levels. A wide numerical investigation was made of some case studies, in order to check the applicability of nonlinear static analyses and propose some new procedures. Nonlinear dynamic analyses have been adopted as reference solution, by using the Proper Orthogonal Decomposition technique in order to catch the dominant behaviours to be compared with those obtained by pushover analysis.