Many alpine and pre-alpine areas of northern Italy are affected by dangerous rockfall phenomena. In designing rockfall protective barriers it is necessary to estimate the reliability of the defensive structures considering the following input parameters: (i) predicted impact energy field, (ii) possible rock block trajectories and (iii) impact strength of the barrier construction material. The reliability analysis aims at predicting the probability that a rock mass may go beyond the barrier (failure or overfly) or beyond the barrier system. The paper describes a design procedure which involves the evaluation of the probability of occurrence of a specific rock body bouncing down. This moving block is characterized by a certain rock mass (m), translation velocity (υt) and rotation velocity (υr). The paper discusses the practical application of a computer block trajectory simulation procedure used to design optimization problems in mountain sites of Friuli which are exposed to rockslide risk.