Bridge Failure Rates, Consequences, and Predictive Trends
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Abstract
A database of United States bridge failures was used to ascertain the failure rate of bridge collapses for a sample population with associated rates by causes. By using the National Bridge Inventory bridge counts, the bridge population, from which the collapsed bridge came from, was determined. The average number of bridge collapses based on the sample population was approximately 1/4,700 annually. The geometric distribution was determined to be a valid model for the number of bridge failures per annum through multiple methods. Based on the data extrapolation and 95% confidence interval, the estimated average annual bridge collapse rate in the United States is between 87 and 222 with an expected value of 128. The database showed hazards that have caused bridges to collapse historically, throughout the United States. Conditional probabilities of collapse rate with consideration for the features under the structures were constructed. The most likely cause of collapse was determined to be hydraulic in nature when adjusting for the features under the structure. The collapse rate of hydraulic causes was unknown from past investigations; however, the value was determined to be an annual rate of 1.52E-4. Collapse rates were also quantifiably established for other causes. The consequences coupled with the rate of failure by cause were quantitatively evaluated. A benchmark, set by the United States Army Corps of Engineers interim guideline for dam safety, was used to show that bridge collapses within the United States are within a tolerable range comparing collapses to life loss. To enhance risk-based and data-driven approaches to bridge management systems in compliance with Moving Ahead for Progress in the 21st Century Act, efficacious bridge collapse data collection is examined for this investigation. Trends obtained from statistical analysis of existing data show 53% of collapsed bridges were structurally deficient prior to collapse, and a failure rate of structurally deficient bridges to be 1/1,100 annually. Age and structural deficiency are related, structural deficiency and collapse are related, and age at collapse is contingent on collapse cause. It was determined that deterioration-caused and overload-caused bridge collapses are age related, but hydraulic-caused and collision-caused bridge collapses are not. Based on the desired results, trends seen in existing collapse data, improved collection efforts and data fields of interest are assessed with recommendations for analytical methods and consequence assessment while maintaining concise data. A national repository of bridge collapses at the federal level is paramount for effective bridge collapse risk analysis. Currently, bridge failure data is incomplete and insufficient to enable in-depth lifetime data analysis for improved bridge preservation. However, the frequency of collapses is often enough for large amounts of data to be collected in relatively few years.