2 Risk-Based and Risk-Informed Design Bases Available to Purchase

Probability of failure or risk-informed assessments are playing an increasingly significant role in defining design load for hazardous facilities [1, 2]. One important role has been in the area of the decision process regarding extreme load design bases. Prior to its employment, hazardous facility design requirements were based, to a considerable degree, on the “minimax” decision rule, which says the worst possible future is sure to be found, so action should be taken to minimize the maximum possible loss. This is popularly referred to as the “what if” basis for design, where loads and particularly their combination, which could be postulated regardless of their probability or conditional probability of occurrence, became design bases.

The minimax rule was tempered somewhat by the reasonable decision rule, which says that any reasonable decision maker under the same circumstances and with the same background would take the same action; hence these decisions tended to be made taking historical precedents into consideration. Finally, there is the acceptable risk decision rule, which say: Take the action where the sum of the products of the probability of exceedence and the consequences of exceedence are equal to or less than a risk associated with some natural phenomena over which society apparently has no control, or is less than some man-made activity that historically has been accepted by society, considering the actual or perceived cost∕benefit to society.

The current tendency in hazardous facility design is a continuing shift away from the minimax rule or deterministic-based siting and design toward an acceptable risk rule tempered with the reasonableness rule. This is typically referred to as risk-informed siting and design. However, it should be understood that there is often an inability to develop an accurate probability assessment in many areas because of the lack of the necessary statistical data or ignorance as to the governing physical relationships and variability in the phenomena being investigated. For this reason deterministic bounds are often placed on the applicable design criteria that are developed based on the acceptable risk rules. Use of probabilities, as a basis of design commonly referred to as risk-based design, must await the development of probability estimates with less uncertainty than is now the case.