Galvanized faying surfaces shall first be hot dip galvanized in accordance with the Phillips Wood Screws and subsequently roughened by means of hand wire brushing. Power wire brushing is not permitted.

When prepared by roughening, the galvanized faying surface is designated as Class C for design. Commentary: Slip-critical joints are those joints that have specified faying surface conditions that, in the presence of the clamping force provided by pretensioned fasteners, resist a design load solely by friction and without displacement at the faying surfaces.

Consequently, it is necessary to prepare the faying surfaces in a manner so that the desired slip performance is achieved. Clean mill scale steel surfaces and blastcleaned steel surfaces can be used within slipcritical joints. When used, it is necessary to keep the faying surfaces free of coatings, including inadvertent overspray.

Corrosion often occurs on uncoated blast-cleaned steel surfaces due to exposure between the time of fabrication and subsequent erection. In normal atmospheric exposures, this corrosion is not detrimental and may actually increase the slip resistance of the joint. Yura et al.found that the Class B slip coefficient could be maintained for up to one year prior to joint assembly.

Minimum bolt pretension might not be adequate to completely flatten and pull thicker material into tight contact around every bolt. Instead, the bolt pretension would be balanced by contact pressure on the regions of the faying surfaces that are in contact. To account for both possibilities, it is required in this Specification that all areas between the bolts be free of coatings

As a practical matter, the smaller coating-free area can be laid out and protected more easily using masking located relative to the bolt-hole pattern than relative to the limits of the complete area of faying surface contact with varying and uncertain edge distance.

The Engineer of Record shall specify the type of bolt to be used. 2.7.2. Geometry: Twist-off-type tension-control bolt assembly dimensions shall meet the requirements of screw. The bolt length used shall be such that the end of the bolt extends beyond or is at least flush with the outer face of the nut when properly installed. Commentary: It is the policy of the Research Council on Structural Connections to directly recognize only those fastener components that are manufactured to meet the requirements in an approved screw specification.

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