Reference

Bridge Engineering Glossary

Plain-language definitions for 31 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-certified inspectors to rate bridge component conditions under the National Bridge Inspection Standards.

Sufficiency Rating

A 0-100 federal formula score used by FHWA to prioritize bridges for repair or replacement funding eligibility.

Bridge Components

Deck Condition Rating

A 0-9 rating of the bridge deck, the surface vehicles drive on, recorded as NBI Item 58.

Superstructure Condition Rating

A 0-9 rating of the structural members that support the deck, including beams, girders, trusses, and arches (NBI Item 59).

Substructure Condition Rating

A 0-9 rating of the foundations and supports beneath the bridge, including abutments, piers, and footings (NBI Item 60).

Classifications

Structurally Deficient

A federal classification for bridges where the deck, superstructure, substructure, or culvert 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 legal loads, recorded as NBI Item 41.

Functionally Obsolete

A retired FHWA classification for bridges built to standards that no longer meet current traffic demands or geometric requirements.

Posted Bridge

A bridge with signed weight restrictions prohibiting vehicles above a specified gross weight from crossing.

Engineering

Scour

Erosion of soil and rock around bridge foundations caused by flowing water, the leading cause of bridge failures in the United States.

Fracture Critical

A bridge containing non-redundant steel tension members whose failure would likely cause partial or complete collapse.

Load Rating

An engineering analysis that determines the maximum live load a bridge can safely carry, expressed as a rating factor.

Scour Critical

A bridge where scour analysis has determined the foundations are unstable for observed or predicted flood conditions (NBI Item 113 code 3).

Fatigue

Progressive cracking of steel members caused by repeated cyclic loading at stress levels below the static strength of the material.

Corrosion

Electrochemical deterioration of metal bridge components, primarily steel reinforcement and structural steel, from exposure to moisture, oxygen, and chlorides.

Prestressed Concrete

A concrete construction method in which steel tendons are tensioned to place the concrete in compression, enabling longer spans and thinner sections.

Steel Girder Bridge

A bridge superstructure using rolled or welded steel I-section beams as primary load-carrying members.

Concrete Girder Bridge

A bridge superstructure using reinforced or prestressed concrete beams as primary load-carrying members.

Truss Bridge

A bridge superstructure composed of triangulated steel or historically timber members forming a rigid framework.

Arch Bridge

A bridge that carries load primarily through compression along a curved arch, transferring forces to abutments or piers at the springings.

Cable-Stayed Bridge

A bridge supported by cables running directly from one or more towers to the deck, carrying load in tension.

Suspension Bridge

A long-span bridge supported by a main cable draped between towers, with vertical hangers suspending the deck below.

Inspection & Data

Average Daily Traffic (ADT)

The average number of vehicles crossing a bridge per day, recorded as NBI Item 29 and used to prioritize maintenance and funding.

National Bridge Inventory (NBI)

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

Federal Highway Administration (FHWA)

The U.S. DOT agency responsible for bridge inspection standards, the National Bridge Inventory, and federal bridge funding programs.

Bridge Inspection

A systematic examination of a bridge's structural components required by federal law, typically every 24 months under the NBIS.

Structure Number

A unique identifier assigned to each bridge in the National Bridge Inventory by the owning state or agency (NBI Item 8).

Inspection Frequency

The interval at which a bridge must be inspected under the National Bridge Inspection Standards, typically 24 months but risk-adjusted under the 2022 NBIS.

Element-Level Inspection

A detailed bridge inspection method that quantifies the condition of individual structural elements using AASHTO-defined element definitions and condition states.

Highway Bridge Program (HBP)

The historical federal funding program for bridge rehabilitation and replacement, now subsumed into IIJA bridge programs and the broader federal-aid highway program.

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).