The Federal Highway Administration provides guidance on those bridges it rates as being “structurally deficient” or “functionally obsolete.”
In terms of providing a database for all bridges on the National Bridge Inventory, the FHWA states that its website is “an information tool. It compiles existing information in a standard format. This tool supports the FHWA’s bridge programs, including the Highway Bridge Program and the National Bridge Inspection Program. The data assists the FHWA in meeting federal responsibilities and reporting requirements and assists the states in meeting national bridge inspection standards. Information entered in the database is collected and maintained with public funds.”
Federal guidelines define a bridge as “structurally deficient” if certain key components―the superstructure, the substructure, or the deck―is rated at 4 out of a possible 10, i.e., “poor,” meaning engineers have identified one or more major defects in its support structure or deck.
Deficient bridges require significant maintenance, rehabilitation or replacement. While not inherently unsafe, states must post limits for both speed and the number of vehicles permitted to travel across these bridges. In short, structurally deficient bridges no longer meet the original design standards for which they were designed and therefore, from an engineering standpoint, have already “failed.”
A “fracture critical” bridge is defined by the FHWA as a steel member in tension, or with a tension element, whose failure would probably cause a portion of or the entire bridge to collapse.
Fracture critical bridges, of which there are a total of about 18,000 throughout the U.S., lack redundancy, which means that in the event of a steel member’s failure there is no path for the transfer of the weight being supported by that member to hold up the bridge. Therefore, failure occurs quickly, as reflected in the video that captured the collapse of the I-35W Bridge in Minnesota.
How are bridges rated?
Federal ratings, which utilize a scale of 1 to 9 (9 meaning in excellent condition and 5 meaning in fair condition), result from overall average condition assessments of a bridge’s three or four major components.
In 1967, the year the I-35W Bridge opened, the collapse of the Silver Bridge in West Virginia spurred the creation of the National Bridge Inspection Program, which mandated routine inspections at least once every two years of bridges that were a part of the federal-aid highway system.
Federal standards for inspections of bridges call for regular observations and measurements that capture the physical and functional state of a bridge. These standards, set out in the American Association of State Highway and Transportation Officials Manual for Condition Evaluation of Bridges, have been adopted by all state transportation agencies.
The inspections, largely close-up visual examinations, are performed by individuals who have met certain qualifications—often as the result of a three-week course under the purview of the FHWA. All inspections include examination of a bridge’s major components: the bridge deck, superstructure, and substructure.
In addition to being subjected to regular conditions inspections, all bridges are analyzed for their capacity to carry vehicular loads. Bridges that cannot safely carry heavy vehicles, such as tractor-trailers, are posted with weight limits. Based upon inspection and load capacity analysis, any bridge deemed unsafe is to be closed.
A comprehensive study performed in 2001 by the FHWA highlighted the fact that visual-only inspections were largely unreliable.
Each state, under federal mandate, is required to perform an annual fracture critical inspection on each bridge in its state inventory. (Fracture critical bridge designs were discontinued in the late 1970s.) Recognizing that fracture critical bridges are inherently lacking the structural capacity to prevent failure if even one structural element fails, the annual fracture critical inspection is intended to identify which of these bridges is increasing its “risk profile” if needed maintenance is not forthcoming. Thus, those fracture critical bridges that have, from continued lack of maintenance to maintain their structural integrity, been rated by state transportation agencies as “structurally deficient,” i.e., “poor” as a reflection of its condition rating, are at the highest level of risk since a failure of any element of such a bridge will trigger a failure mode and potential catastrophe for all located on or near such a bridge.