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 slipcritical 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 offers a defined slip-coefficient. Other paints that do not offer a defined slip-coefficient are not permitted in areas closer than one bolt steelwise MAY 2015 steelwise 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.

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 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. Roughened (see 6.7.4) hot-dip galvanized surfaces also provide a Class A slip coefficient, μ=0.30.

6.7.4. As required in the RCSC Specification Section 3.2.2(c), galvanized surfaces in slip-critical connections must be roughened by means of hand wire brushing. What treatment constitutes roughening?

The Guide to Design Criteria for Bolted and Riveted Joints (published by RCSC) indicates that the galvanized surface must be visibly altered without disrupting the continuity of the galvanizing. This is usually accomplished by wire brushing as indicated in the RCSC Specification Section 3.2.2, such treatment must be controlled to achieve the necessary roughening or scoring. Power wire-brushing is generally not acceptable because it tends to polish the surface rather than roughen it. Note that an acceptable result can be achieved with a variable-speed power tool with a stiff wire brush when used at a speed that is comparable to that for hand wire brushing.