Understanding "Structurally Deficient" In Bridge Ratings

Staff | September 28, 2010

Conn Abnee, executive director of the National Steel Bridge Alliance (NSBA), a non-profit association that promotes the use of steel bridges, says that "structurally deficient" and "functionally obsolete" may not be as bad as they sound, when used in bridge ratings.

According to Abnee, the Federal Highway Administration (FHWA) uses these terms as two of many variables in a formula to determine the funding it provides for bridge-replacement projects within states.

FHWA, he says, calculates an overall sufficiency rating for each bridge by combining more than 30 characteristics.

The FHWA will fund 80 percent of a bridge's replacement cost if the bridge has a sufficiency rating of 50 or less (out of 100 possible points) and is considered either structurally deficient or functionally obsolete. Bridges with higher ratings do not qualify for federal funding.

Functionally obsolete bridges are generally older and have roadway geometry that doesn't match today's standards. A functionally obsolete bridge may be structurally sufficient, but unable to handle its current volume of traffic.

Structurally deficient bridges, says Abnee, may be lacking in any one of five areas, as determined by inspection. Every two years bridge inspectors from state DOTs evaluate five bridge characteristics to report their conditions: the deck (generally the concrete roadway), superstructure (usually girders supporting the deck), substructure (vertical piers supporting the superstructure and deck), structural evaluation (a calculated value comparing load-carrying capacity to traffic volume), and waterway adequacy (generally, clearance above high water).

Examples of bridge conditions leading to a rating of structurally deficient:

  • a spalled, cracked concrete deck
  • corrosion, deterioration, cracking and chipping, or erosion of concrete bridge piers
  • removal of foundation support by erosion or localized scour by water
  • corrosion or cracking of concrete or steel girders
  • vertical or horizontal bridge movement affecting structural stability
  • occasional overtopping during high water, causing significanttraffic delays.

Depending on the condition that triggered the "structurally deficient" rating, says Abnee, a bridge may not necessarily require immediate structural repair or load restrictions. Also, since structural deficiency is only one part of the overall sufficiency rating, a structurally deficient bridge does not necessarily qualify for federal funding for replacement.

Additionally, Abnee says, it is misleading to use the classification "structurally deficient" to compare bridge materials or types.

In response to the Minneapolis bridge tragedy, the NSBA has created a task force of its designing, fabricating and engineering members to recommend ways to accelerate the designing and streamline the construction to rehabilitate or replace qualifying bridges.