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

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NCHRP 24-25 Page 1 Phase II Final Report EXECUTIVE SUMMARY Bridges are skillfully designed to withstand the most common natural hazards (e.g. floods, erosion/scour, earthquakes), but the inherent uncertainty associated with natural phenomena requires bridge owners to regularly inspect bridges for signs of a problem (i.e. vulnerability to failure). While some aspects of a bridge’s vulnerability to failure are easy to inspect (e.g. visible cracks or corrosion), the condition of a bridge’s foundation is comparatively difficult and expensive to inspect and evaluate using standard methods. Analysis of National Bridge Inventory (NBI) data, as published in 2005, shows that almost half of the States in the US had more than a thousand bridges with unknown foundations. This is disturbing since the percentage of bridges with unknown foundations supporting principal arterials (high traffic roads) in a given State ranges from 0 to 25%. It is equally troubling that more than 1,500 bridges with unknown foundations nation-wide have been built since the year 2000. The first phase of this study surveyed the expert opinion of various specialists including various engineers, economists, and State transportation officials. This analysis generally showed that risk-based methods provide the most inexpensive and flexible way to select a management plan. These methods generally use available data to estimate the monetary risk associated with a particular failure, and then weigh this against the cost of various mitigating actions (e.g. increased monitoring, foundation reconnaissance, countermeasures or retrofits). Estimating risk of failure involves correlating historic rates of failure with the potential for a given hazard at a given site and with uncertain indicators of the bridge’s vulnerability to failure. The risk equation used in this study is the product of the estimated probability of failure and the total cost of failure. The total cost of failure is

NCHRP 24-25 Page 2 Phase II Final Report the sum of the cost of replacing the bridge, the costs of lost time and additional mileage on detours, and the cost of loss of life (in the event of failure). Given the uncertainty in such risk estimates, the general approach to risk management outlined in this report suggests a series of three consecutive screening analyses to select the most appropriate management plan. The first screen states that high priority bridges – bridges that provide access to emergency services, evacuation routes, or support principal arterials – should automatically qualify for the most aggressive management plan (i.e. foundation reconnaissance required to perform standard failure analyses). The second screen involves setting minimum performance levels (MPL) for various functional classifications (NBI item 26). Any remaining bridges with unknown foundations with an estimated probability of failure greater than its pertinent MPL should also receive the most aggressive management plan. The third screen involves comparing the estimated risk of failure for any remaining bridges to the cost of installing automated monitoring, then to the cost of installing countermeasures, to see if any of these special activities are warranted. If countermeasures are warranted, then automated monitoring is probably not warranted. Similarly, if countermeasures are warranted but the cost of foundation reconnaissance and standard failure analyses is more than 50% of the cost of the countermeasures (or retrofits), then foundation reconnaissance and standard failure analyses may not be warranted before installing the countermeasures. If countermeasures are installed without standard failure analysis, then the engineer should install countermeasures that are appropriate for similar site and bridge types. This general approach was then applied specifically to scour failure by using the scour vulnerability assumptions proposed in the HYRISK methodology. Analysis shows that there is a strong correlation between HYRISK’s estimated scour vulnerability and the known scour vulnerability of 297,796 bridges with known foundations. The probability of

NCHRP 24-25 Page 3 Phase II Final Report scour failure was estimated by contacting transportation officials about historical scour failures nation-wide. Twenty-five States provided data via phone interviews and emails, and this data suggests that the annual average probability of failure is 33/161,000 = 0.000205, or about 1 in 5,000 per year. Scaling this to all bridges over water (i.e. 379,788) yields almost 80 scour failures per year. Applying the original HYRISK method to all of the bridges over water in the NBI database yields about 60,511 failures per year (i.e. the sum of the individual probabilities of failure). Since these assumptions clearly do not correspond with experience and result in exaggerated risk factors, all of the original HYRISK failure probabilities were scaled down to a level corresponding to the approximate number of failures (nation-wide) obtained from the State interviews (i.e. about 100 scour failures nation-wide per year). Statistical rules are used to estimate a lifetime probability of failure from the annual probability of failure and the tentative remaining life of a bridge. The report then summarizes the cost and suitability of various state-of-the-art scour mitigation methods including foundation reconnaissance via non-destructive testing, scour monitoring equipment, and scour countermeasure designs. The general guidelines for risk management of bridges with unknown foundations were then customized for scour failure. Minimum performance levels were selected for different functional classifications (i.e. NBI item 26) such that all arterials perform at least as good as the national average annual probability of failure (0.0002). The scour management guidelines show the engineer how to estimate probability of scour failure and the cost of appropriate scour monitoring methods, scour countermeasures, non-destructive testing, and scour analyses. The guidelines also show the engineer how to perform the cost- benefit analyses already described. The scour guidelines were then applied to data from sixty case studies from six States in order to validate the overall approach. Twenty-nine of the case studies involved

NCHRP 24-25 Page 4 Phase II Final Report bridges with known foundations and scour evaluations, and two of the case studies involved bridges that actually failed due to scour. The results show that most of the twenty-nine case studies (including the scour failures) for which there are known scour evaluations validate the management plan that the scour guidelines suggested. There were only three case studies – collector-classed bridges – with known foundations for which the scour guidelines may not have recommended a sufficiently aggressive management plan. This possibility for error is the primary reason why the minimum requirement in the scour guidelines is to develop a bridge closure plan, and to keep a detailed record of the stream bed’s elevation during biennial inspections. Monitoring the stream bed elevation every two years and reviewing/updating the bridge closure plan each time should help officials identify problems that may not have been apparent before this risk analysis. 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). 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. Once the general approach has been developed for other hazards (earthquakes, tsunamis, etc.), the joint probability of failure due to multiple hazards may be estimated collectively.