“…Subsequently, the ultimate load limit, occurs at force F m and deformation δ m , after which the response deteriorates, up to a residual force level, denoted with force F r and deformation δ u . with the initial stiffness, ini S , is taken equal to 0.5, based on the characterization of experimental data [21] and available analytical modeling [4]. For the needs of estimating the ultimate displacement u , the negative stiffness of the third branch of the force-deformation law has been approximated in a range of 0.01-0.1 of initial stiffness [22], with value 0.1 corresponding to quite brittle masonry with weak framing.…”