In this paper, an important feature of time variant reliability problems is identified. The concept of barrier failure dominance (BFD) is introduced to characterize those problems where an out-crossing or overload failure is more likely to be caused by a very small realization of the barrier (resistance) than by an exceptionally large realization of the load process. This result arises from a study of the ensemble out-crossing rate approximation, reported in a previous paper and which is reviewed herein. It is shown how the notion of BFD can be used to understand the limits of application of the ensemble out-crossing rate approximation. It is also suggested that the acknowledgement of BFD can support the choice of the appropriate simplified solution method for particular problems. A three-dimensional load combination and a resistance degradation problem are studied as examples. The ensemble up-crossing rate approximation is compared with load process-based approximations, using the time-integrated approach, the load combination rule of Turkstra and the point-crossing formula.
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