Freak Events, Black Swans, and Unknowable Unknowns: Impact on Risk-Based Design

Abstract

To design means making informed decisions about suitable alternatives in the face of uncertainties. As a result, structural design criteria and inspection plans reflect the objective of satisfactory performance under well selected extreme conditions. The extent to which the extreme boundary is “pushed” depends on the design approach (ex: component vs system design), the nature and the consequences of the hazards, and risk acceptance, all of which fit neatly into the traditional framework of decision theory. This basic framework is also broad enough to include wider socio-economic and environmental objects, so that provisions with respect to robustness, resilience, sustainability, and risk mitigative measures in general, can be effectively accounted for. Various civil engineering fields suffer from a perception that we don’t dig deep enough, that we fail to consider “beyond extreme” scenarios. Every major accident, or any exceptional natural disaster, or any surprising combination of circumstances, triggers a new call for re-examination of the design rationale: if a freak event can be explained, then surely it should be (have been) accounted for. This paper looks at what really lies beyond our “design frontier”. We distinguish between three broad classes of events: far-out extremes for heavy-tailed hazards, scenarios marked by very unlikely combinations of events (perfect storms), and so-called unknowable unknowns. We identify, from a decision making point of view, which objectives, which tools, and which risk measures can be used, and which lessons can be learned.