Structurally Deficient and Functionally Obsolete
The Federal Highway Administration has established definitions for the terms below. Structural deficiency ratings are based on the National Bridge Inventory ratings scale.
Structurally Deficient (SD) Status - A highway bridge is classified as structurally deficient if the deck, superstructure, substructure, or culvert is rated in "poor" condition (0 to 4 on the NBI rating scale). A bridge can also be classified as structurally deficient if its load carrying capacity is significantly below current design standards or if a waterway below frequently overtops the bridge during floods.
Functionally Obsolete (FO) Status - Highway bridges classified as functionally obsolete are NOT structurally deficient, but their design is outdated. They may have lower load carrying capacity, narrower shoulders or less clearance underneath than bridges built to the current standard.