Fault tree analysis of bridge failure due to scour and channel instability

Fault tree analysis is a systematic method of analyzing events that lead to an undesirable event. Fault trees can be used to assess the probability of failure of a system (or top event), to compare design alternatives, to identify critical events that will significantly contribute to the occurrence of the top event, and to determine the sensitivity of the probability of failure of the top event to various contributions of basic events. In this paper, fault tree analysis is used to determine the probability of failure of a bridge due to scour and other geomorphic channel instabilities. This analysis permits an examination of a very complex system of interactions and processes that are not well understood. Minimal knowledge regarding the actual processes of scour and channel instability is required for a fault tree analysis. Three examples of analyses for actual bridges are provided showing different combinations of scour (local and contraction) and geomorphic instabilities (channel widening and degradation) at both the abutments and piers. Riprap placed at the abutments for protection is also included in the analyses.