The Need for Hierarchies of Acceptance Criteria for Probabilistic Risk Assessments in Fire Engineering

A probabilistic risk assessment (PRA) is commonly accepted as a tool for performance based design in fire safety engineering, but the position of PRA in the design process, the relationship between different acceptance concepts (absolute, comparative, ALARP), and the responsibilities of the designer remain unclear. Aiming to clarify these aspects, the safety foundation of fire safety solutions is investigated, indicating that PRA is necessary for demonstrating adequate safety when no appeal can be made to the collective experience of the profession. It is suggested that PRA is not a methodology for ‘future fire safety engineering’, but rather a necessary methodology to provide an objective safety foundation for uncommon fire safety designs. Acknowledging that what constitutes ‘acceptable safety’ is subjective and may change over time, an objective proxy of ‘adequate safety’ is defined and proposed as a benchmark against which to assess the adequacy of fire safety designs. In order to clarify the PRA process, a hierarchy of different acceptance concepts is presented. Finally, it is shown how, depending on the applied acceptance concepts, the designer takes responsibility for different implicit assumptions regarding the safety performance of the final design.