What are the changes affecting HDG in the AASHTO LRFD Bridge Design Specification (10th edition)?

The American Association of State Highway and Transportation Officials (AASHTO) via the Committee on Bridges & Structures (CoBS) recently released the 10th edition of its LRFD Bridge Design Specifications for use in the design, evaluation, and rehabilitation of bridges nationwide. The AASHTO Steel & Metals Committee (formerly T14 Committee on Steel Structures) approved ballot items revising Section 6 on Steel Structures directly pertaining to hot-dip galvanized (HDG) bridge projects.

Creep Reduction Factor for HDG

In Article 6.13.2.8, A creep reduction factor K_c = 0.80 was introduced into the equation for determining the nominal slip resistance (R_n)of a galvanized faying surface (Class C, µ = 0.30) or duplex-coated faying surfaces utilizing a coating applied over hot-dip galvanizing to produce a higher slip coefficient (Class D, µ = 0.45). For all other surface conditions, K_c = 1.

This revision is related to commentary language in RCSC Specification for Structural Joints Using High-Strength Bolts which says coating thicknesses over 15 mils [380 μm] on each mating surface may creep under sustained bolt tension, reducing clamping force and slip resistance. While RCSC and AGA continue to research this topic, AASHTO adopted a conservative 20% reduction to avoid the potential for concern over limits on HDG coating thickness for faying surfaces. The specifications for HDG do not require a maximum coating thickness.
The revision will result in a slight increase in the number of bolts used in HDG slip-critical connections.

Slip-Critical Connections & Mixed Faying Surfaces

For bridge projects containing a mixture of larger steel fabrications (plate girders too large for HDG) with smaller or more complex components suitable for HDG (cross frames, cable stay tubes, railings, and more), a tandem coating of HDG and metallizing has been successfully demonstrated for maximum cost efficiencies while leveraging the benefits of both coating systems. Examples include the I-95 overpass at Fairfield Avenue bridge over I-95 (CT), the Kosciuszko Bridge (NY), and the I-90 overpass at Commonwealth Ave. (MA).

Previously, slip-critical connections involving a galvanized faying surface and a metallized faying surface were not readily described by the AASHTO standards. However, they could be qualified to one of the surface condition classifications in accordance with Appendix A of RCSC’s Specification for Structural Joints Using High-Strength Bolts and used if approved by the Engineer. Testing can take several months and is subject to laboratory availability.

New in the 10^th edition (Article 6.13.2.8), connections involving a hot-dip galvanized surface mated to a metallized faying surface are now classified as a Class D surface condition for slip (µ = 0.45) without the need for additional cost and planning for project-specific testing. Some limitations on metallizing composition and coating thickness are applicable in the new definition of Class D.

Class D Surface: blast-cleaned surfaces with Class D coatings, or a mixed faying surface utilizing an unsealed pure zinc or 85/15 zinc/aluminum thermal-sprayed coating with a thickness less than or equal to 16 mils mating with a hot-dip galvanized surface.

Although a Class D (µ = 0.45) surface condition has a slightly lower slip coefficient value than Class B (µ = 0.50), the difference between Class D and B does not cause a significant impact in the overall number of bolts required. Another benefit of this revision is a greater variety of options for the specifier/designer to optimize on cost and corrosion protection.

Note: AGA and the International Zinc Association are currently developing a new Specifiers Guide for the projects pairing hot-dip galvanizing alongside zinc thermal spray.