Steel Solutions Center

6.7. Faying Surfaces

6.7.1. When is paint permitted on the faying surfaces of bolted connections?

In snug-tight and fully-tensioned bearing connections, paint is unconditionally permitted on the faying surfaces. In slip-critical connections, however, if paint is present, it must be a qualified paint. A qualified paint is one that has been tested in accordance with the RCSC Specification Appendix A and provides a method to determine a slip-coefficient. Other paints that do not offer a defined slip-coefficient are not permitted in areas closer than one bolt diameter but not less than 1 in. from the edge of any hole and in all areas within the bolt pattern of slip-critical connections, even when due to inadvertent over-spray. Coating slip requirements in Appendix A were revised in the 2020 edition of the RCSC Specification. Among many changes the certification of slip resistance is valid for 84 months. The degree of cure of the coating is an essential variable.

6.7.2. Both the AISC and RCSC Specifications require that paint on the faying surfaces of slip-critical connections be qualified (providing a minimum slip coefficient) or that such surfaces remain unpainted. Does this requirement apply to the surfaces und

No. In a slip-critical connection, the faying surfaces are those that resist the relative movement (or slip) of the plies. This occurs on the contact surfaces between the plies, not those surfaces under the bolt head or nut. 

6.7.3. What is the difference between the surface preparation requirements for Class A and B surfaces in slip-critical connections?

With uncoated faying surfaces, clean mill scale provides a Class A slip resistance, μ=0.30, whereas blast cleaning is required to obtain the higher Class B slip coefficient, μ=0.50. With painted faying surfaces, the slip resistance is determined by the tested performance of the paint system as meeting Class A, B, or some other intermediate slip coefficient and the steel to be painted must be blast-cleaned in all cases. Hot-dip galvanized surfaces also provide a Class A slip coefficient, μ=0.30. The requirement to roughen hot-dip galvanized surfaces has been relaxed.

6.7.4. Do galvanized surfaces in slip-critical connections require roughening by means of hand wire brushing?

Prior versions of the RCSC Specifications included a requirement to hand wire-brush galvanized surfaces in slip-critical joints. However, recent research combined with current galvanizing practices has revealed that such brushing does not improve slip resistance capacity and may actually reduce capacity. Section 5.4 in the RCSC Specification designates galvanized faying surfaces as Class A for design.