The paper deals with the shear failure mode with diagonal cracking of masonry panels in existing buildings. More specifically, this study is focused on the b shape factor of the failure criterion included in the current Italian technical Recommendations (NTC 2008) and completes the results obtained in a companion paper of the same authors /7/. Three plane unreinforced masonry walls with regular openings are considered with different slenderness of the masonry beams. The walls are modelled by finite elements and by equivalent frames and then subject to pushover analyses. In the framed models the b shape factor has been selected both according to the NTC 2008 and as proposed in the companion paper /7/. The seismic capacity diagrams of the walls show that the equivalent framed models significantly overestimate the ultimate shear of the walls with respect to the results provided by the finite element models. This result is amplified when the b values are assumed as recommended in the NTC 2008.