6.10. Washer Requirements
To assure proper bolt performance, it is required in the RCSC Specification Section 6.1.1 that the surfaces against which the head and nut bear have a slope not greater than 1:20 with respect to the plane normal to the bolt axis. American standard beams (Sshapes) and channels are rolled with beveled flanges that exceed this limit. Because bolt holes are made perpendicular to the outside face of these flanges, a beveled washer must be used at the inside face to provide the required parallelism. Beveled washers are made square or rectangular so that they can more easily be prevented from turning to assure that the bevel is oriented in the proper direction
Two reasons for this requirement are to prevent “dishing” of the washer (which would become more critical if an edge were unsupported) and to prevent moisture from entering the connection (thus creating a corrosion concern).
For long-slotted holes, thicker plate washers are required per RCSC Specification Section 6.2.5 because the high clamping pressures generated during pre-tensioned installation and the increased amount of material removed with a long-slotted hole can result in collapse of thinner standard F436 washers into the hole.