Frequently Asked Questions

Q. What does bridge sufficiency rating mean?

A. The sufficiency rating formula is a method of evaluating factors that indicate a bridge’s sufficiency to remain in service. The result of the formula is a percentage in which 100 percent represents an entirely sufficient bridge and zero percent represents an entirely insufficient or deficient bridge. The sufficiency rating is never less than 0 or more than 100.
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Q. What is the age and condition of interstate bridges?

A. The Interstate System has 55,315 bridges. Of that number, 17.4 percent were constructed during the 1950s, 44 percent in the 1960s, and 20 percent in the 1970s.

While bridge conditions have improved over the past decade, some 8,841 interstate bridges are rated as structurally deficient. Exhibit 11-9 shows that approximately 15.9 percent of all rural interstate bridges were deficient in 2004, including 1,163 that were structurally deficient (about 4.2 percent of the total number) and 3,224 that were functionally obsolete (11.7 percent of the total number).

Among rural functional systems, the rural Interstate System had the lowest percentage of structurally deficient bridges and lowest number of functionally obsolete bridges and deficient bridges.



The percentage of deficient bridges has steadily declined in recent years. In 1994, 18.5 percent of rural Interstate bridges were structurally deficient. That percentage had declined to 15.9 percent by 2004. The percentage of structurally deficient urban interstate bridges also declined, from 30.6 percent in 1994 to 26.5 percent in 2004.

Q. What is the National Highway System?

A.The National Highway System (NHS) of the United States comprises approximately 160,000 miles of roadway, including the Interstate Highway System as well as other roads, which are important to the nation's economy, defense, and mobility. The NHS was identified in 1995 by the U.S. Department of Transportation's Federal Highway Administration in cooperation with the states, local officials, and metropolitan planning organizations.

The National Highway System (NHS) comprises only 4.1 percent of the nation's total road mileage, but carries 44.8 percent of vehicle traffic. There are 115,104 bridges on the NHS, which includes the interstate bridges. Of these, 5.6 percent are considered structurally deficient, down from 7.9 percent in 1995.

In 2004, 19.4 percent of all U.S. bridges were located on the NHS, but these bridges had 49.5 percent of the total deck area of all bridges and carried 71.1 percent of the traffic on all bridges. Approximately 20.5 percent of NHS bridges were considered structurally deficient in 2004, including 5.6 percent classified as structurally deficient and 14.9 percent classified as functionally obsolete.

In 2004, all levels of government spent a combined $34.6 billion for capital improvements to the NHS, which was 49.2 percent of total capital expenditures on all roads.

Q. Which bridges are included in the National Bridge Inventory (NBI) system?

A. NBI structures are bridges or culverts that are open to the public, carry vehicular traffic and have an opening longer than 20 feet, measured along the center of the roadway.

Q. What bridges are not considered part of the Nation Bridge Inventory (NBI) system?

A. Non-NBI structures include bridges or culverts that carry vehicular traffic and are equal to or less than 20 feet at the center of the bridge.

Q. What is a "structurally deficient"bridge?

A. Structurally deficient means there are elements of the bridge that need to be monitored and/or repaired.  The fact that a bridge is "structurally deficient" does not imply that it is likely to collapse or that it is unsafe. It means the bridge must be monitored, inspected and

repaired/replaced at an appropriate time to maintain its structural integrity.

Q. How is "structural deficiency" determined?

A. The condition of different parts of a bridge is rated on a scale of 0 to 9 (with 9 being “excellent” and zero being “failed”). A structurally deficient bridge is one for which the deck (riding surface), the superstructure (supports immediately beneath the driving surface) or the substructure (foundation and supporting posts and piers) are rated in condition 4 or less.

Q. What makes a bridge structurally deficient, and are structural deficient bridges unsafe?

A. The fact that a bridge is "structurally deficient" does not imply that it is likely to collapse or that it is unsafe. A “deficient” bridge is one with some maintenance concerns that do not post a safety risk. A “deficient” bridge typically requires maintenance and repair and eventual rehabilitation or replacement to address deficiencies. To remain open to traffic, structurally deficient bridges may be posted, if necessary, with reduced weight limits that restrict the gross weight of vehicles using the bridges. If unsafe conditions are identified during a physical inspection, the structure must be closed.

Q. What is a “functionally obsolete” bridge?

A. A functionally obsolete bridge is one that was built to standards that are not used today. These bridges are not automatically rated as structurally deficient, nor are they inherently unsafe. Functionally obsolete bridges are those that do not have adequate lane widths, shoulder widths, or vertical clearances to serve current traffic demand or to meet the current geometric standards, or those that may be occasionally flooded.

Q. What is a “fracture-critical” bridge?

A. A fracture-critical bridge is one that does not contain redundant supporting elements. This means that if those key supports fail, the bridge would be in danger of collapse. This does not mean the bridge is inherently unsafe, only that there is a lack of redundancy in its design.

 

References:

Federal Highway Administration's 2006 Conditions and Performance Report