Issue 43

A. Luciani et alii, Frattura ed Integrità Strutturale, 43 (2018) 241-250; DOI: 10.3221/IGF-ESIS.43.19 249 ' (1 ) r r N C N    where r N is the number of blocks reaching the road without the protection device, ' r N those reaching the road with the device and ܥ is the catching capacity of the device. The catching capacity is the percentage of blocks that the device can stop. The requested catching capacity should be evaluated through the trajectories analysis and the design block assessment. The residual efficiency proposed above describes the energy the damaged barrier can withstand compared to the nominal one. Therefore, it can easily be taken into account in the risk analysis modifying the previous equation as follows ' (1 ) r ef r N C r N     The residual efficiency reduces the catching capacity, i.e. increases the number of blocks reaching the road ( ' r N ), with an obvious increase of the rockfall risk. Therefore, in the design, the effect of a damaged barrier can be considered in the trajectories analysis simulating a barrier with a reduced ability to stop a certain energy and allowing more blocks to go through. C ONCLUSIONS he influence of damages induced by ageing on the behaviour of a rockfall protection net fence is analyzed using numerical modelling. A site survey on many net fence installations located in North of Italy allowed to point out the most relevant problems related to ageing of rockfall protection net fences after installation and suggested how to set up the numerical models. The main goal of the analyses is to show how damages of the components can affect the efficiency of the products and therefore reduce their ability to stop falling blocks. To develop this assessment six different numerical models of a commercial net fence have been studied and an assessment of the residual efficiency has been developed. This value may be included in rockfall risk analysis allowing to take into account the conditions of a damaged or aged device. In this procedure the residual efficiency value should be used to reduce the catching capacity of the net fence increasing the number of falling blocks that might impact against the structure to be protected and consequently correctly considering in the risk analysis the presence of aged net fences on the slope. It is important to highlight that all the risk analysis procedures have as an input datum the number of blocks impacting the structure. Moreover, this appraisal allows owners to plan maintenance or refurbishment works and establish priority between different protection devices, knowing when the reduction of efficiency induces a risk higher than the accepted threshold value. R EFERENCES [1] Budetta, P., Assessment of rockfall risk along roads, Nat. Hazards Earth Syst. Sci., 4 (2004) 71–81. [2] Corominas, J., Copons, R., Moya, J., Vilaplana, J.M., Altimir, J., Amigò, J., Quantitative assessment of the residual risk in a rockfall protected area, Landslides, 2 (2005) 343-357. [3] Peila, D., Guardini, C., Use of the event tree to assess the risk reduction obtained from rockfall protection devices, Nat. Hazard Earth Syst. Sci., 8 (2008) 1441-1450. [4] Mignelli, C., Lo Russo, S., Peila, D., ROckfall risk MAnagement assessment: the RO.MA . approach, Nat. Hazards, 62 (2012) 1109-1123. [5] De Biagi, V., Botto, A., Napoli, M.L., Dimasi, C., Laio, F., Peila, D., Barbero, M., Estimation of the return period of rock falls according to the block size, Geoingegneria Ambientale e Mineraria, 147 (2016) 39-47. [In Italian] [6] Budetta, P., Nappi, M., Comparison between qualitative rockfall risk rating systems for a road affected by high traffic intensity, Nat. Hazards Earth Syst. Sci., 13 (2013) 643-1653. [7] Mignelli, C., Peila, D., Lo Russo, S., Ratto, S., Broccolato, M., Analysis of rockfall risk on mountainside roads: evaluation of the effect of protection devices, Nat. Hazards, 73 (2014) 25-35. T