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What are the new changes for slip critical connections in the AASHTO LRFD Bridge Design Specification (8th edition)?

The AASHTO Load Resistance Factor (LRFD) Bridge Design Specifications are intended for use in the design, evaluation, and rehabilitation of bridges, and are mandated by the Federal Highway Administration (FHWA) for use on all bridges using federal funding. The Technical Committee for Structural Steel Design (T14) recently passed changes to Section 6 (Steel Structures) of the document and the revised language will appear in the 8th revision to be published Fall 2017.

The revised language of Section 6 reflects the need within the industry to re-define the available surface condition classes and associated values for slip coefficients used to calculate the nominal slip resistance of a high-strength bolt in slip-critical connections. 

Summary of Revisions

Class definitions within the 11th paragraph of Article C6.13.2.8  were updated to revise the class for hot-dip galvanized faying surfaces, and include options for metallized faying surfaces and blast-cleaned surfaces coated with zinc-rich paints:

  • Class A:  Unpainted clean mill scale and blast-cleaned surfaces with Class A coatings. The value for a Class A surface condition is reduced from 0.33 to 0.30 to better align with the value provided in the latest RCSC specification corresponding to the median of available historical data.
  • Class B:  Unpainted blast-cleaned surfaces to SSPC-SP 6 or better, blast-cleaned surfaces with Class B coatings, or unsealed (pure zinc or 85/15 zinc/aluminum) thermal-sprayed coatings with a thickness less than or equal to 16 mils. Unsealed thermal-spray coatings were not previously addressed by the specification, but are now included within Class B based on recent research data.
  • Class C:  Hot-dip galvanized surfaces. Based on recent industry research, the value for a Class C surface conditions is reduced from 0.33 to 0.30, and subsequent treatment (wire brushing) of the galvanized surface is no longer required.
  • Class D:  Blast-cleaned surfaces with Class D coatings.  Added to increase the options for organic zinc-rich coatings over any blast cleaned surface (including HDG).

Updated slip coefficient values for each class are reflected in Table 6.13.2.8-3, and a summary of the overall changes are as follows:

Surface ConditionDefinitionKs (Slip Coefficient)
Class A
  • Unpainted clean mill scale
  • Blast-cleaned surfaces with Class A coatings
0.30
Class B
  • Unpainted blast-cleaned surfaces to SSPC-SP 6 or better
  • Blast-cleaned surfaces with Class B coatings
  • Unsealed (pure Zn or 85/15 Zn/Al) thermal-sprayed coatings with a thickness ? 16 mils
0.50
Class C
  • Hot-dip galvanized surfaces (roughening by wire brushing no longer required)    
0.30
Class D
  • Blast-cleaned surfaces (including HDG) painted with organic zinc-rich coatings
0.45

How Do These Changes Impact the HDG industry & Bridge Design?

Eventually, specifications related to structural connections used in other industries may be similarly revised.  Meanwhile, for the galvanizer involved in any federal highway and transportation projects, wire brushing of the HDG faying surface is no longer required.  Doing so will only lead to increased cost, as industry research determined wire brushing does not increase slip properties.

For the FHWA/bridge customer, the revisions allow a greater variety of coating systems that can be used for the design of high-strength slip-critical connections. Specifically, it will become easier and cheaper for the specifier to select hot-dip galvanizing and metallizing for corrosion protection. Although the slip coefficient for hot-dip galvanized surfaces is reduced from 0.33 to 0.30 in this revision based on the results of industry research, it is anticipated the new value will have minimal impact on design. However, there is a  potential for a small increase in the number of bolts used in connections with HDG fasteners. Regardless, customers will benefit from the removal of additional labor previously required to roughen HDG faying surfaces. For the new Class D surface condition, a slightly lower slip coefficient value is provided than for Class B, but the value will not cause a significant impact in the overall number of bolts required for most high-strength bolted connections. Therefore, the addition of Class D simply provides a greater variety of coating options to the specifier/designer, including the use of HDG surfaces with zinc-rich paints.

For more information on the latest revision to the AASHTO LRFD Bridge Design Specifications or hot-dip galvanized slip-critical connections, please contact the AGA Technical Department.


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