appear in the higher frequencies and can be damped numerically with appropriate time-stepping schemes, such as by employing the damped Newmark [35]. Note that in the Newmark's algorithm, the artificial damping introduced in the time-stepping procedure is controlled by the parameter δ: a value of δ¼1/2 corresponds to an undamped formulation, while by increasing δ41/2 the artifi-cial damping can be influenced [36]. The parameters α and δ can be chosen independently, however, in the case of symmetric positive-definite matrices, the method is unconditionally stable for