Risk-Informed Approaches to Safety Regulation - Structural Integrity of Offshore Wind Turbines - page 135

lic finds acceptable. In the vast majority of studies to date involving
structural performance and reliability, the term “risk” is used more or
less interchangeably with “probability” or is thought of as the comple-
ment of “reliability” (Ellingwood 1994). Consequences (e.g., economic
losses; morbidity and mortality) are included only indirectly, if at all; low
target probability goals are typically assigned, somewhat arbitrarily and
on the basis of judgment, to high-consequence events. While current
codes and standards as well as code enforcement keep failure rates at a
low level, no one knows exactly what a socially acceptable failure rate for
buildings, bridges, and other structures might be, although structural
engineers believe that current codes and standards deliver civil infra-
structure with risks that are acceptable in most cases. At the other
extreme, the de minimis risk below which society normally does not
impose any regulatory guidance is on the order of 10 −7 /year (Paté-Cornell
1994). Failure rates for buildings, bridges, dams, and other civil infra-
structure that may be calculated through the use of classical reliability
analysis (Ellingwood 2000) fall in a range between 10 −3 /year and 10 −7 /year,
a gray area within which risk-reduction measures are traded off against
increments in the cost of risk reduction. The notion of having risks “as
low as reasonably practicable” (Stewart and Melchers 1997), which is
common in industrial risk management, is based on this concept. In
sum, what constitutes acceptable risk is relative and can be established
or mandated only in the context of what is acceptable in other activities,
what investment is required to reduce the risk (or socialize it), and what
losses might be entailed if the risk were to increase.
The following section considers how the general concepts of risk assess-
ment and management summarized above have been implemented for
several types of civil infrastructure. The unique nature of each infrastruc-
ture type determines how specific risk-informed decision concepts have
been implemented.
PROBABILITY-BASED LIMIT STATES DESIGN
Load and Resistance Factor Design
Structural codes and standards applicable to the design of civil infra-
structure traditionally have been concerned primarily with public safety
(preventing loss of life or personal injury) and, in this context, the collapse