Risk Criteria

Because decisions concerning risk are strongly influenced by the criter used to measure the risk present, it is important to select an appropriate and intuitively-clear measure. The commonly-used return period (average length of time between hazardous events) is often misleading. A clearer criterion is the encounter probability, which is the probability that a structure will encounter a hazardous event during its useful life. It is decidedly dangerous to design for a hazardous event having the same return period as the expected life. (e. g., there is 0.636 probability that a structure with a 50-yr life will encounter a hazardous event having a 50-yr return period). Other useful criteria, such as the probability distribution of the total damage, the expected total damage, and the probability of zero damage are based on estimates of the damage as a function of the hazard event intensity and on the density of the hazard events during the different seasons of the year. The choice of the most appropriate criterion is unavoidably a matter of engineering judgment.