Superstructure: The parts of a bridge that are above the bottom of the girders. Girders, bridge deck, and bridge railing are parts of the superstructure.

Truss Bridge: A bridge (such as the I-5 bridge over the Skagit River) composed of rigid, jointed structures that support longer spans.

Condition designations

Fracture critical: Bridges receive this designation if they are designed with certain load-bearing parts of the superstructure that would cause the bridge to collapse or be unable to function if they should fail.

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Structurally deficient: A bridge receives this designation if it is in a structurally deteriorated condition and does not adequately carry its designed traffic loads. The rating is applied if a bridge meets one of the following condition codes: superstructure deck, and/or substructure rates at 4 out of 10, (poor condition) or less. Or, if any of the appraised codes for structural adequacy or waterway adequacy are at 2 out of 10 (substandard) or less. Weight restrictions or closures may be posted depending on the limits of the bridge’s load carrying capacity.

Functionally obsolete: A bridge receives this designation because it either cannot meet current traffic demands or does not meet current design standards. The rating means the bridge does not have adequate approach, alignment, geometry, clearance, structural adequacy, or water adequacy to meet intended traffic needs. The rating is applied if any of the measures mentioned above are rated 3 or below on a scale of 10.

Sufficiency rating: This measures the bridge’s relative capability to serve its intended purpose. Ratings will vary from 0 to 100.

— Hal Bernton, Seattle Times staff reporter

Sources: Washington Department of Transportation and Oregon Department of Transportation