Reference

Bridge Engineering Glossary

Plain-language definitions for 16 bridge engineering, inspection, and safety terms.

Condition Ratings Bridge Components Classifications Engineering Inspection & Data

Condition Ratings

Condition Score

A 0-100 rating that summarizes a bridge's overall structural health based on NBI inspection data.

NBI Rating Scale (0-9)

The standard 0-9 scale used by FHWA inspectors to rate bridge component conditions.

Sufficiency Rating

A 0-100 score used by FHWA to determine a bridge's eligibility for federal repair or replacement funding.

Bridge Components

Deck Condition Rating

A 0-9 rating of the bridge deck — the surface vehicles drive on — based on visual inspection.

Superstructure Condition Rating

A 0-9 rating of the structural members that support the deck, including beams, girders, and trusses.

Substructure Condition Rating

A 0-9 rating of the foundations and supports beneath the bridge, including piers and abutments.

Classifications

Structurally Deficient

A federal classification for bridges where any major component is rated 4 or below on the 0-9 NBI scale.

Load Posting

A weight restriction placed on a bridge when its load-carrying capacity falls below standard design limits.

Functionally Obsolete

A bridge that was built to standards that no longer meet current traffic demands or design requirements.

Engineering

Scour

Erosion of soil and rock around bridge foundations caused by water flow, a leading cause of bridge failure.

Fracture Critical

A bridge with steel tension members whose failure would cause partial or complete bridge collapse.

Inspection & Data

Average Daily Traffic (ADT)

The average number of vehicles crossing a bridge per day, used to assess usage and prioritize maintenance.

National Bridge Inventory (NBI)

The federal database of all bridges on public roads in the United States, maintained by FHWA.

Federal Highway Administration (FHWA)

The federal agency responsible for bridge inspection standards and maintaining the National Bridge Inventory.

Bridge Inspection

A systematic examination of a bridge's structural components required by federal law at least every 24 months.

Structure Number

A unique identifier assigned to each bridge in the National Bridge Inventory by the owning state or agency.

Frequently Asked Questions

What is the difference between structurally deficient and functionally obsolete?

A structurally deficient bridge has a component rated 4 or below on the 0-9 NBI scale, meaning it needs significant repair. A functionally obsolete bridge was built to standards that no longer meet current traffic demands — it may be structurally sound but too narrow or have insufficient clearance for modern traffic.

How is a bridge Condition Score calculated?

The Condition Score is a 0-100 rating based on three NBI component ratings: deck condition (33%), superstructure condition (33%), and substructure condition (34%). Bridges over 50 years old receive an age penalty deduction. Scores map to letter grades: A (80-100), B (60-79), C (40-59), D (20-39), F (0-19).