The design of net fences as passive mitigation measures against rockfall events has represented a challenge since the last decades. The choice of the proper effect of the actions to consider in the design is still under debate. Recently, the Authors have proposed a novel time-independent reliability approach encompassing the large variability of the size and the kinematics of the possible impacting blocks. The entire statistics of all these quantities (size, velocity, and height) enters into the calculations, differently from other approaches that consider specific values of the parameters. In addition, the variability in time of the inputs is tackled, with particular reference to the size of the falling block. The recent approach is herein merged and compared with the current semi-probabilistic ultimate limit state design approach, suggested in the Eurocodes and implemented in the Italian recommendations UNI 11211-4:2018, with the purpose of finding the equivalent partial safety factors of kinetic energy and trajectory height of the impacting block. A sensitivity analysis with different synthetic profiles, representing possible real situations, is performed highlighting that if a set of partial safety factors is assigned to different sites, an intrinsic variability in the failure probability has to be accepted.
Rockfall represents one of the major hazards in mountain environment [1, 2] where the implementation of effective structural mitigation measures is often required [3, 4, 5]. Installing net fences is a possible solution for risk management. Different solutions to realize these systems, defined as construction protection kits, have been proposed, improving the adopted components, material, and the assembly [4, 6, 7]. As a consequence, the efficient design of these structural measures has been of particular interest, even though a codified design solution has not been defined yet. UNI 11211-4 [8] in Italy and ONR 24810 [9] in Austria constitute the existing national standards, only.
The prescriptive-based design for each component, according to which standards on material, configuration, strength and stiffness have to be guaranteed, is not feasible for complex and unique structural systems [10]. On the contrary, the performance-based design [11] has been often adopted for assembled structures, allowing more comprehensible design requirements, and, thus, the quality and innovation of the systems. This design approach is based on the premise that structural systems have to achieve specific performance objectives and, thus, the definition of the end goal is the starting point of the design process. A hybrid solution is adopted for net fences. The producers identify optimal new technologies and solutions and assess the complying performance through numerical analyses [12, 13, 14, 15, 6, 16, 17, 18] and testings, according the requests of EAD 340059-00-0106 [19]. In details, the performance is related to the interception and stopping of falling blocks with a height and kinetic energy. Once installed on a slope, due to the variability of the conditions and the random nature of the phenomenon, the barrier can experience a multitude of impacts different from the ones for which the kits were tested. Hence, prescriptive-based approaches are thus necessary to guarantee an adequate level of safety, which is measured through the reliability of the system. The reliability-based design consists in a procedure which allows considering the random nature of the parameters describing a structure and the actions on it [20]. In the framework of the ultimate limit state design suggested by EC7 [21], the following inequalities must be verified:
(Equation)