Safety, Reliability and Structural Design

In the safety analysis of any structure the expected operational life for which it is, or should be designed, represents an important design parameter, the significance of which is not sufficiently understood. The concept of design for infinite life is as unrealistic as that of design for zero probabtlity of failure, since the design life and the probability of failure are related through the “reliability function” of the designed structure. By expressing the probability of survival as a function of operational life, this function permits a rational approach to the safety analysis of modern structures for the criteria of ultimate load failure and of fatigue, as well as for the combination of both criteria. In reality the safety factor is shown to be a statistical distribution function of the quotient of resistance and load considered as statistical variables. No engineering significance can, therefore, be attached to any specific single value of this factor other than as a “measure of location” (mean, median, made) of the statistical distribution. However, the probability or risk of failure or of survival is a unique parameter of the distribution function of the safety factor, and should replace the conventional concept of safety or rather be considered as its only rational quantitative measure. Its generalization in the form of the “reliability function” provides the basis for an integrated safety analysis of structures permitting quantitative differentiation between the safety of non-redundant and redundant structures.