Augmented System Level Failure Events for Bridges under Earthquake Hazards

Abstract

Bridge level failure event definitions per limit state have evolved from failure of one key component as representative of the system, to failure of at least one of many components. In practice, this and similar definitions limit the inclusion of joint failures of several components in a previous limit state, whose effects on repair cost and functionality can equate to system failure in the next limit state. This paper explores a closed form combinatorial method to evaluate all possible ways in which components can fail and argues that typical system failure definitions per limit state can incorporate joint failures of multiple important components in a previous limit state. Other failure events can be considered for the augmentation of typical system events, but this work focuses on the most influential components because they can be visually identified through conditional importance measures as a by-product of the combinatorial approach. Bridges in as-built circumstances and retrofitted with elastomeric bearings are used to illustrate the augmentation calculation, which increases the median system fragility at moderate limit states in the range of 4% to 20%. This methodology to connect bridge component to system reliability can readily support infrastructure stakeholder decision making and risk management.