To explain the terms used in Structural Engineer’s September 2007 issue, both in my "From the Editor" column and in a news story, the following is a partial statement submitted by National Steel Bridge Alliance Executive Director Conn Abnee.—Jennifer Goupil, P.E.

Understanding the phrase ’structurally deficient’

The recent collapse of the bridge in Minneapolis has focused the public’s attention on the classifications "structurally deficient," as well as "functionally obsolete," as applied to bridges. The terms have highly negative connotations, but what do they really mean?

The Federal Highway Administration (FHWA) uses these terms as two variables in a formula to determine funding levels for bridge replacement projects within states. FHWA calculates an overall bridge sufficiency rating to appraise the nation’s bridges. Calculations of sufficiency ratings are generally a function of more than 30 bridge characteristics, depending on the state.

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

Functionally obsolete bridges are generally older bridges that have roadway geometry conditions that are not in compliance with today’s standards. A bridge considered functionally obsolete may be structurally sufficient, but unable to handle the volume of current traffic demands.

Every two years, bridge inspectors from state DOTs evaluate five bridge characteristics to report their conditions: the deck, superstructure, substructure, structural evaluation, and waterway adequacy. A bridge deemed lacking in any one of these five areas may be classified structurally deficient.

Depending on precisely what condition triggered the structurally deficient rating, a bridge may not necessarily require immediate structural repair or load restrictions. Also, since this condition is only one component of the overall sufficiency rating, a structurally deficient bridge does not necessarily qualify for federal funding for replacement.