Risk-Based Management Guidelines for Scour at Bridges with Unknown Foundations (2007)

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NCHRP 24-25 Page 81 Phase II Final Report 7. CONCLUSIONS AND RECOMMENDATIONS This research reveals a number of important facts concerning bridges with unknown foundations and managing their potential vulnerability to unexpected failure. „ Bridges with unknown foundations are prevalent in many states. Many of them are old structures, but 1,506 have been constructed between 2000 and 2004. „ A bridge’s foundation may differ considerably from its design plan. Thus, if as- built construction records are lost, then the bridge’s vulnerability to hazards that degrade or stress the foundation can not be properly evaluated without expending funds to determine the foundation. „ Experts can correlate pertinent bridge failures (or estimates of potential failures) with relevant data that is easily obtained for bridges with unknown foundations in order to estimate probability of failure. „ The sixty case studies regarding scour failure in this report show that risk of failure (i.e. probability*cost) can be successfully used to identify bridges that warrant special activities (e.g. automated monitoring, countermeasures or retrofits, replacement, or closure). „ Given the uncertainty with these estimates, this study also shows that it is prudent to establish performance standards (maximum probability of failure) that are a function a bridge’s importance (i.e. functional classification). „ While most of the analysis in this report focuses on estimating a bridge’s vulnerability to scour failure, the general approach outlined here should be applicable to many other hazards (e.g. earthquakes, debris flows, tsunamis, etc.). The “Scour Risk Management Guidelines” in this report admittedly benefit from the collective research and experience of many private, state, and federal institutions. The analysis presented in the “Annual Probability of Scour Failure” section focuses on using

NCHRP 24-25 Page 82 Phase II Final Report existing data to estimate scour vulnerability and probability of failure, which is clearly useful but subject to significant uncertainty. Thus, future studies of scour vulnerability should focus on relating scour vulnerability to better indicators, which may not be currently monitored but cost less than performing foundation reconnaissance on thousands of less- important bridges with unknown foundations that may be low-risk. It is important that this research focus on improving predictions of both a site’s potential for scour (i.e. hazardous potential) as well as the bridge’s vulnerability to failure (i.e. structural “weakness”). Other hazards – like earthquakes, debris flows, tsunamis, etc. – are less common and thus harder to study and counteract. The “General Approach to Risk Management” section of this report provides a useful outline for how future research projects can begin the work of correlating pertinent bridge failures (or estimates of potential failures) to relevant indicators of hazardous potential and vulnerability to failure. The scour research presented in this report is a valuable example of the general approach. Once this has been developed for other hazards, the joint probability of failure due to multiple hazards may be estimated collectively.