National Bridge Inventory
The National Bridge Inventory (NBI) is a database, compiled by the Federal Highway Administration, with information on all bridges and tunnels in the United States that have roads passing above or below. This is similar to the grade crossing identifier number database compiled by the Federal Railroad Administration which identifies all railroad crossings. This bridge information includes the design of the bridge and the dimensions of the usable portion. The data is often used to analyze bridges and judge their conditions. The bridge inventory is developed with the purpose of having a unified database for bridges, including identification information, bridge types and specifications, operational conditions, bridge data including geometric data and functional description, inspection data, etc. Any bridge more than 20 feet (6 meters) long used for vehicular traffic is included.
Identification information addresses the bridge location uniquely and classifies the type of the routes carried out on and/or under the structure and locates the bridge within the spatial location. Each bridge is given a number by the highway department of the respective state or the agency that maintains the bridge. The method of assigning numbers differs from one state to the next, but provides a unique number for each bridge in the state.
The bridge inventory is developed for having a unified database for bridges including the identification information, bridge types and specifications, operational conditions, and bridge data including geometric data, functional description, inspection data, etc. Identification information addresses the bridge location uniquely and classifies the type of the routes carried out on and/or under the structure and locates the bridge within the spatial location. Bridge type and specifications classify the type of the bridge. This part provides defined standard categories for classification of the bridges. It also identifies the material of the bridge components, deck and deck surface. Operational conditions provide information about the age of the structure as well as construction year, rehabilitation year, type of services and traffic carried over and/or under the structure number of the lanes over and/or under the bridges, average daily traffic, average daily truck traffic and information regarding to bypass, detours. Furthermore, the bridge inventory contains information regarding to inspection data, ratings assigned by inspectors and appraisal results.
Structural evaluation scale
The National Bridge Inventory includes a structural evaluation of deck, superstructure, and substructure, on a 0-9 scale:
- 9 - Superior to present desirable criteria
- 8 - Equal to present desirable criteria
- 7 - Better than present minimum criteria
- 6 - Equal to present minimum criteria
- 5 - Somewhat better than minimum adequacy to tolerate being left in place as is
- 4 - Meets minimum tolerable limits to be left in place as is
- 3 - Basically intolerable requiring high priority of corrective action
- 2 - Basically intolerable requiring high priority of replacement
- 1 - This value of rating code not used
- 0 - Bridge closed
It also classifies bridges as either "structurally deficient" or "functionally obsolete". Neither necessarily implies lack of safety, though that could be a concern as well. "Structurally deficient" means that the condition of the bridge includes a significant defect, which often means that speed or weight limits must be put on the bridge to ensure safety; a structural evaluation of 4 or lower qualifies a bridge as "structurally deficient". The designation can also apply if the approaches flood regularly. "Functionally obsolete" means that the design of a bridge is not suitable for its current use, such as lack of safety shoulders or the inability to handle current traffic volume, speed, size, or weight. 
In December 2008, 72,868 bridges in the United States (12.1%) were categorized as "structurally deficient", representing an estimated $48 billion in repairs, and 89,024 (12.2%) were rated "functionally obsolete", representing an estimated $91 billion in replacement costs.
Some bridges are also identified as "fracture critical", which means that (usually due to low-cost design) the failure of any one main structural member will cause a significant portion or the entire bridge to collapse. Such designs were discontinued in the 1970s. Fracture critical designs can leave bridges vulnerable to collisions with ships or large trucks, as in the I-5 Skagit River Bridge collapse.
Issues and usage
The NBI was subjected to scrutiny and questions after new actions revealed it might be outdated or inaccurate. Federal officials attempted to order emergency inspections of all steel truss bridges after a major bridge collapse in Minneapolis in 2007. However, it was found that many records within the NBI were inaccurate or out of date.
The Inventory is used for federal funding purposes. A "bridge sufficiency rating" is calculated, based 55% on the structural evaluation, 30% on the obsolescence of its design, and 15% on its importance to the public. As of 2008, a score of 80 or less is required for federal repair funding, and 50 or less for federal replacement funding.
- "AASHTO - Bridging the Gap". Transportation1.org. 2008-03-17. Retrieved 2012-06-02.
- "Bridges | Report Card for America's Infrastructure". Infrastructurereportcard.org. Retrieved 2012-06-02.
- Svirsky, Alexander. "National Bridge Inventory Database". Retrieved 2014-12-12.
- Bridge collapse revealed holes in federal data By Bill Dedman, msnbc.com, Friday, August 1, 2008.
- msnbc.com's Bridge Tracker interactive map based on the newest National Bridge Inventory
- msnbc.com investigation: Late inspections of bridges put travelers at risk
- National Bridge Inventory (Data available in ASCII)
- National Bridge Structural Deficiency Data by State/Year Built (Data Available in Excel)
- National Bridge Inventory search (by Alexander Svirsky)
- Navigating the National Bridge Inventory