Published June 5, 2025
How Bridge Condition Grades Work: A-F Ratings Explained
BridgeSafety assigns every bridge in America a letter grade from A to F based on its condition data from the FHWA National Bridge Inventory. These grades translate the technical 0-9 NBI rating scale into an intuitive format that anyone can understand. Here is exactly how the grading system works and what each grade means for the bridges you cross every day.
The Three Components
Every bridge in the NBI receives separate condition ratings for three primary structural components, each evaluated on a 0-9 scale during routine inspections:
- Deck condition (33% of grade) — The deck is the driving surface that vehicles travel on. Inspectors evaluate cracking, spalling, potholes, delamination, exposed rebar, and drainage.
- Superstructure condition (33% of grade) — The superstructure includes beams, girders, trusses, and other elements that support the deck and transfer loads to the substructure. Inspectors check for corrosion, fatigue cracking, section loss, and connection deterioration.
- Substructure condition (34% of grade) — The substructure consists of piers, abutments, and foundations that transfer loads to the ground. Key concerns include settlement, tilting, scour, and cracking.
The Scoring Formula
BridgeSafety converts the three NBI component ratings into a composite score between 0 and 100:
- Each NBI rating (0-9) is converted to a percentage: (rating / 9) x 100
- The three percentages are weighted: deck (33%), superstructure (33%), substructure (34%)
- For bridges over 50 years old, an age penalty is applied that reduces the score, reflecting the increased risk associated with aging infrastructure
- The final composite score maps to a letter grade
Grade Breakdown
| Grade | Score Range | NBI Equivalent | Meaning |
|---|---|---|---|
| A | 90-100 | 7-9 | Excellent to very good condition. Minor maintenance only. |
| B | 80-89 | 6-7 | Good to satisfactory. Some minor deterioration noted. |
| C | 70-79 | 5-6 | Fair condition. All elements functional but showing wear. |
| D | 55-69 | 4-5 | Poor condition. Advanced deterioration, repair needed. |
| F | 0-54 | 0-4 | Serious to critical. Major structural issues, may be restricted. |
Why We Use Letter Grades
The FHWA's 0-9 scale is precise but not intuitive. Telling someone a bridge has a "deck rating of 5" does not convey meaning the way "Grade C" does. Letter grades make bridge condition data accessible to the public, journalists, elected officials, and community advocates who need to understand infrastructure health without engineering expertise.
The ASCE Infrastructure Report Card uses a similar A-F approach to grade America's overall infrastructure. Our grades apply the same concept to individual bridges using the NBI's detailed inspection data.
Grade Distribution Across America
The current distribution of bridge grades across all 624,000+ bridges in the NBI reveals a bell curve centered around B and C grades:
- Grade A: Approximately 25% of bridges — mostly newer structures built after 2000 or recently rehabilitated
- Grade B: Approximately 35% — the largest group, representing bridges in good working condition with routine maintenance
- Grade C: Approximately 25% — bridges showing their age but still functional
- Grade D: Approximately 10% — bridges needing significant repair work
- Grade F: Approximately 5% — the most deteriorated structures, many already weight-restricted or closed to traffic
You can explore grade distributions by state on our rankings page or look up the grade for any specific bridge in our database.
Limitations of the Grading System
No single grade can capture every aspect of a bridge's condition and safety. The BridgeSafety grade focuses on structural condition but does not directly account for factors like load capacity, seismic vulnerability, or functional obsolescence. A bridge can receive a good structural grade but still be too narrow, have inadequate clearance, or lack modern safety features. We encourage users to read the full bridge report for a complete picture.
Frequently Asked Questions
A grade of A means the bridge is in excellent or very good condition across all three components (deck, superstructure, substructure). The NBI ratings for all components are 7 or above on the 0-9 scale, indicating only minor maintenance needs.
A grade of F means one or more bridge components are in serious, critical, or failed condition (NBI rating 3 or below). These bridges have major structural deterioration and may be closed or severely weight-restricted. Immediate action is typically required.
BridgeSafety calculates a composite score from three NBI condition ratings: deck (33%), superstructure (33%), and substructure (34%). Bridges over 50 years old receive an age penalty. The composite score maps to letter grades: 90-100 = A, 80-89 = B, 70-79 = C, 55-69 = D, below 55 = F.
No. The FHWA uses a 0-9 numeric scale for individual bridge components. BridgeSafety combines these into a single composite score and translates it to an A-F letter grade for easier public understanding. The underlying data is the same NBI data.
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