Critical Industrial Facilities: Simply Applying Current Importance Factors γ_I is not Enough!

Abstract

A generic seismic risk study for critical industrial facilities (CIFs) is presented and discussed in detail. The study is focussed on the residual seismic risk of critical facilities supposed to be correctly designed according to Eurocode (EC) 8. The initial objective was to define a design importance factor γ_I in order to achieve sufficiently low probabilities of a major accident. The residual seismic risk is dominated by earthquakes for which the probability of occurrence is typically one or two orders of magnitude lower than for the design earthquake. According to Swiss practice, the annual probability of a major accident with more than 100 fatalities outside the industrial facility must not exceed 10-7. In order to achieve this goal, it turned out that a design earthquake with a return period of the order of 100’000 years should be considered, with an associated importance factor around 8! Such a design, however, would be technically and economically unfeasible. Therefore, it is necessary to adopt a risk based view and first explore all possibilities of reducing the largest possible number of fatalities – by other means than just a strong seismic design. At present, it is not yet clear what will be done by the Swiss authorities once all reasonably practicable measures of reducing the size of the largest possible accidents have been put into action and the residual seismic risk is still too high. In any case, however, it is strongly recommended to also look at what could happen if GMs much above design GM occur, instead of simply design for a given GM level.