Sep 4 2007
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 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% 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.
What about structurally deficient ratings? 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). A bridge deemed lacking in any one of these five areas may be classified structurally deficient.
Examples of bridge conditions leading to a classification of structurally deficient include:
- 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 significant traffic delays.
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.
Additionally, it's highly misleading to use the classification structurally deficient to compare bridge materials or types. Location of the deficiency (deck, superstructure, substructure, or foundation) is not stated.
Aside from creating confusion about the term structurally deficient, the Minneapolis bridge tragedy has spotlighted the need to improve national infrastructure. In response, the NSBA has created a task force of our design, fabricating, and engineering members to evaluate and make recommendations on two issues: accelerating the design work and streamlining the construction process for the rehabilitation or replacement of qualifying bridges.
Our members have the hands-on, practical experience of building and repairing bridges around the country. This know-how will be useful in determining what works, what doesn't, and how the systems for building, inspecting, and maintaining bridges can be made more efficient and cost- effective without compromising public safety.