The 10th Edition of the AASHTO LRFD Bridge Design Specifications introduces important updates directly impacting hot-dip galvanized (HDG) steel in bridge construction. Approved by the AASHTO Steel & Metals Committee, these changes primarily affect slip-critical connections and the classification of galvanized and metallized faying surfaces. Understanding these revisions is critical for engineers, fabricators, and contractors working on highway and transportation infrastructure.
Why the Update Matters
Bridge structures often combine multiple steel fabrication methods. While large girders may not fit in a galvanizing kettle, smaller bridge elements—like cross frames, railings, and cable stay tubes—are frequently galvanized for corrosion protection. The new revisions bring clarity and consistency to design practices involving HDG in such mixed environments.
For full technical details, see the original AGA article on HDG in the AASHTO LRFD 10th Edition.
Creep Reduction Factor for HDG
A new creep reduction factor (Kc = 0.80) has been introduced for galvanized faying surfaces (Class C, μ = 0.30) and duplex-coated faying surfaces (Class D, μ = 0.45).
This adjustment accounts for the potential reduction in slip resistance due to thicker coatings (>15 mils) under sustained bolt tension. While typical HDG coatings fall between 3–8 mils, the specification conservatively applies a 20% reduction factor. In practice, this may result in a modest increase in the number of bolts required for slip-critical connections involving HDG.
Mixed Faying Surfaces: Galvanized & Metallized
One of the most notable changes is the formal recognition of mixed faying surfaces—where one side is hot-dip galvanized and the other is metallized.
In the new specification (Article 6.13.2.8), these connections are now classified as Class D with a slip coefficient of μ = 0.45. Class D applies when an unsealed zinc or zinc-aluminum thermal-sprayed coating (≤16 mils) mates with an HDG surface.
While this coefficient is slightly lower than the Class B value (μ = 0.50), the difference has minimal impact on the overall number of bolts required. Importantly, this classification eliminates the lengthy testing and approval process previously required for such configurations.
Real-World Applications
The updated guidance validates practices already used in landmark bridge projects, such as:
- I-95 Overpass at Fairfield Avenue (CT)
- Kosciuszko Bridge (NY)
- I-90 Overpass at Commonwealth Avenue (MA)
These projects leveraged tandem systems of galvanizing and metallizing to maximize durability and cost efficiency.
The 10th Edition of the AASHTO LRFD Bridge Design Specifications brings greater clarity and consistency to the use of hot-dip galvanizing in bridge projects. The introduction of a creep reduction factor and the classification of galvanized-metallized faying surfaces will help streamline design decisions, reduce testing delays, and ensure reliable long-term performance of bridge connections.
For detailed specification language and commentary, refer to the AGA's technical article.
