The meaning of Beta: background and applicability of the target reliability index for normal conditions to structural fire engineering

Uncommon structural fire engineering designs must demonstrate adequate safety, in principle, through a balancing of the uncertain future costs and benefits of safety investments. In structural design for normal load conditions ('ambient design') this level of detail is commonly avoided through the application of reliability targets. In order to inform the development of reliability targets for structural fire design, the background of the ambient reliability targets is discussed. It is found that different common ambient reliability targets are broadly comparable when taking into account differences in assumptions and applications. As recent reliability targets have been informed by simplified cost-optimizations, the derivation of such a model is presented. The derivation allows identification of possible pitfalls when extending ambient reliability targets to structural fire design. It is concluded that ambient safety targets cannot readily be scaled as a function of the fire occurrence rate for application to structural fire engineering problems. The underlying cost-optimization model is, however, applicable as a concept. A number of issues requiring further attention are identified.