For the last 14 years flexible rockfall barriers have been improved considerably as far as their energy absorbing capacity is concerned. In design concepts a rock's kinetic energy is compared with the energy absorbing capacity of the retaining structure. The former can be estimated by rockfall modelling and the latter by full scale testing of rockfall barriers. Important questions conceming the structural safety can, however, not be answered by this method. A more sophisticated approach which considers the acting forces and the behaviour of the structural components is necessary. The aim is to build a barrier that can safely absorb its required energy and still retains as much of its original height as possible. For this purpose, a certain stiffness is essential. This, of course, has consequences on the forces acting on the system and its elements. It is quite clear, that the forces have to be kept within reasonable limits.
Analysing the results of our rockfall tests and applying the mechanical laws that govern the relations between impact velocity, deceleration intervals and the corresponding braking distances, we can assess the peak forces acting on structural elements. The deceleration factor f_k (force factor f_k) introduced in this context and described in this paper is an important number to characterise structural behaviour.