Slip Coefficient Testing of ASTM A709 Grade 50CR Steel and Dissimilar Metal Slip-Critical Bolted Connections

About the report:

The purpose of this study was to conduct slip coefficient testing of bolted connections made from ASTM A709 Grade 50CR steel (hereinafter“50CR steel”) to determine how they fit into the current AASHTO LRFD Bridge Design Specifications (hereinafter “the AASHTO BDS”) surface condition classifications.  Currently, 50CR steel s not included in these classifications because it was not being used for bridges when the existing surface condition classifications were developed in the 1980s.  The slip coefficient tests conducted in this study included both compression slip and tension creep tests as specified by the Research Council of Structural Connections.  The test specimens consisted of both uniform and dissimilar metal connections; uniform connections were made up entirely of 50CR steel, and dissimilar metal connections were made up of both 50CR steel and either weathering or galvanized steel. Dissimilar metal connections were included in the testing because their future use is anticipated in which a bridge girder would be constructed using both 50CR and other ASTM A709 bridge steels. In these cases, 50CR steel would be used in highly corrosive parts of abridge, such as near joints or close to water, and either weathering or galvanized steel would be used in less corrosive parts of the bridge to provide cost savings.  In addition to different steel types, the test specimens included various surface finishes, including unblasted, steel shot blast-cleaned, and garnet blast-cleaned surfaces.  In total, 55 slip tests and 33 creep tests were conducted as part of this study. 

The test results showed that unblasted 50CR steel has a slip coefficient value of at least 0.30, which meets the current AASHTO BDS Class A surface condition for unblasted steel.  Additional testing should be conducted to determine if unblasted 50CR steel could provide a greater slip coefficient value.  If additional testing shows this to be true, using a greater slip coefficient for unblasted 50CR steel would provide additional cost savings since blast cleaning would not be necessary to reach a greater slip coefficient value.  Blast-cleaned 50CR steel from either steel shot or garnet media has a slip coefficient value of 0.50, which meets the current AASHTO BDS Class B surface condition for blast-cleaned steel.  When dissimilar metal connections are made with 50CR steel, the design slip coefficient value of the connection can be taken equal to the smaller of the two slip coefficient values being jointed. 

The study recommends that the Virginia Transportation Research Council conduct a technical assistance study to determine the appropriate sample size for slip coefficient tests on 50CR steel and dissimilar metal bolted connections that is necessary to recommend changes to the surface classification and slip coefficient value of 50CR steel in the AASHTO BDS and the Virginia Department of Transportation bridge design specifications.  

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