Diagonal cracking shear strength of unreinforced masonry panels: a correction proposal of the b shape factor

After summarising the failure criteria adopted by the new Italian Seismic Code (NTC 2008) for the seismic assessment of unreinforced masonry panels (URM), the paper presents a numerical study aimed at investigating the b shape factor. This factor is a coefficient, function of the panels’ slenderness, employed to evaluate the ultimate shear strength of URM for the failure mechanism with diagonal cracking. The results herein presented show that the actual values of the coefficient b are higher than those proposed by the NTC (2008); consequently, the shear strength obtained by applying the Italian Seismic Code is not conservative. An amendment is proposed for the b shape factor, and its effects are evaluated through the analysis of three plane-URM walls with regular openings and different slenderness of the masonry beams. Pushover analyses were performed to estimate their seismic capacity and their collapse modes. The walls were modelled by both the finite element method (FEM) and the equivalent frame approach (EFM). In the EFM approach the b shape factor was selected both according to the NTC (2008) and as proposed in the paper. The seismic capacity curves show that the EFM approach significantly overestimate the ultimate shear strength of the walls with respect to the results obtained by the FEM, and this effect is amplified when the b shape factor is evaluated as recommended by the NTC (2008).