Bridge Condition Forecasts, Two Ways

Bridges are expensive to build and expensive to maintain. They represent large investments and are crucial to keep wheels turning and businesses open. According to the American Society of Civil Engineers’ 2017 infrastructure scorecard, America’s bridges scored a C+ grade. Of the 614,387 bridges in the nation, almost 4 in 10 are 50 years or older and just over 56,000 are structurally deficient.

Top five US states with the most structurally deficient bridges

This is why bridge asset management is important: to secure a good return on investments with timely and carefully selected intervention projects and to prolong the useful life of each bridge while keeping it in a state of good repair. The key to determining which intervention projects to perform and when, is to accurately forecast the future condition of a bridge.

Condition forecasts can be performed in the AgileAssets system in two ways. One way is by using the NBI component ratings (on a scale from 0 to 9) for the Deck, Superstructure, and Substructure. The other is by using the element condition state distribution (i.e. the varying quantities of an element in one or more of the four condition states CS1, CS2, CS3, and CS4) for all elements – NBE (national bridge elements), BME (bridge management elements), and ADE (agency developed elements) – in a bridge.

Forecasting using the NBI ratings for bridge components is done using a performance curve that indicates a relationship between attributes such as the age, traffic, and other parameters and the condition of each component. The system uses this performance curve to project the future condition per year for a specified time duration, e.g. 10 or 20 years from the current year. Based on this projected future condition ratings, the system determines and generates projects from a predefined set to counter declining conditions and bring the component condition to a desired condition state, which prevents further deterioration. This approach, however, does not result in a detailed set of recommended projects, since the focus is on just the three major components of the structure.

NBI rating classification for bridge components

Bridge performance over time

Forecasting using the element condition state distribution is performed for each individual bridge element. Forecasting for some duration in the future is done by applying a transition matrix – that allows for a set or varying lengths of deterioration from one condition state to the next – to the current condition state distribution. The forecasted future condition state distribution of each bridge element can then be aggregated into a set of health indices based on a predefined grouping of like elements, (e.g. deck health index, railing health index) Based on the future condition states of bridge elements as well as the health indices, the system determines and generates projects to counter a declining element or index. This approach, given the more detailed inspection data available for analysis, results in a more detailed set of project recommendations.

Employing one of these methods to forecast the condition of bridge components or elements, ultimately allows a bridge engineer to generate intervention strategies to prolong the useful life of these critical assets and maximize return on the large investments made to build them.