PSI - Issue 78

Marco Faggella et al. / Procedia Structural Integrity 78 (2026) 2141–2146

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A time-adaptive representation can be defined, which weighs reliability scenarios as a function of time. The basis for this process is well detailed in NTC18-C Section C.3.2.1, which introduces the risk and reliability principle of equal probability of exceedance P VR related to the V R parameter with the purpose of adaptively referring failure likelihoods to a specific facility reference life and class of use. Based on the same principle we can decide to downscale the reference time to the shorter periods domain. This would allow us to distinguish between the domain of Design-oriented lifetime of a facility (normally in the range of 50 or 100 years), and the domain of Short-term Operation and risk Decision timeframe (normally in the range of 3 to 5 years). At SLC, the code Section C.3.2.1 procedure can be visualized drawing a horizontal 5% PoE threshold line corresponding to V R =100 years (SLC New Design code mandated objective) to intersect the lowered performance vulnerability PSHA line corresponding to 30% PGA in the smaller periods range. Analytically this is done entering the Poisson equation of NTC18-C C.3.2.1 with the 30% PGA return Period T R30% and solving it by the new short reference time. It must be noted that for probabilities as low as 10% and 5% (SLV and SLC), the Poisson equation (1) can be approximated at first order providing the decision time (time to failure) t p ≈ T R30% *P VR , which for SLC yields a value as high as 7 years. This is depicted in the log PSHA plot. In this case, multi-risk uncertainty quantification showed that at such low operating reserve, hydraulic risk may outrange seismic risk by 10 to 20 times and therefore should be prioritized. This indicates that a short term (3 to 5 years) risk-seeking earthquake approach could prevent precautionary drawdowns and allow to rise the storage height limitation. 6. Conclusions This study presents a probabilistic multi-risk framework grounded in NTC18-compliant PSHA and equal probability of exceedance principles for evaluating infrastructural performance under overlapping seismic and non seismic risks. The methodology integrates four interrelated components: a Poisson-based time-to-failure model, an earthquake risk model derived from PSHA, a reliability model for environmental risks (storage loss), and a comparative reliability framework capable of adapting across decision timeframes. Together, these elements allow for a coherent, interoperable representation of risk that can be applied to both short-term decisional contexts (5-10 years) and long-term structural design scenarios (50-100 years reference life design). By using Poisson probability processes and time-convolution models based on annual exceedance rates, the framework supports consistent reliability comparisons across distinct failure modes. It quantifies the divergence between lower and upper bounds of risk, offering a structured means to complement engineering judgment (vulnerability and safety factor reduction) with probabilistic insight. Importantly, the method allows infrastructure managers to make informed decisions in contexts where hazards interact or evolve over time, such as in climate affected or earthquake-prone regions. The application of this framework to the Camastra Dam illustrates the limitations of narrowly applied regulatory policies. Although the dam’s seismic vulnerability warranted caution, the imposition of a drastic reservoir restriction — limiting storage to one-third of its capacity — resulted in a substantial increase in the probability of water supply failure. This storage loss probability, estimated at 20% within a five-year operational window, exceeded the likelihood of most plausible seismic collapse or high damage scenarios. Acknowledgements The support of University of Porto FEUP CONSTRUCT ICS and of Prof. Humberto Varum is gratefully acknowledged. References

Basson, M. S., & Van Rooyen, J. A., 2001. Practical application of probabilistic approaches to the management of water resource systems. Journal of Hydrology, 241(1-2), 53-61.