As a minimum, standard cut washers should be used with bolts to keep a bolt head or nut from causing crushing when tightening is taking place. Where a steel plate is used, the head or nut bears directly on steel, and the washers are not required.
Common types of washers are shown in Figure 5.11 below.
If square or round steel plate washers are used, they must be of adequate thickness to prevent cupping and overstressing of the steel. Round plate washers may be used instead of square plate washers for appearance reasons such as for exposed trusses. Bevelled washers are necessary where the bolts are not perpendicular to the bearing surface.
Minimum dimensions for washers used in timber connector joints depend on the type of washer and size of the bolt and connector (split rings and shear plates), and are specified in engineering design standards (CAN/CSA-O86.1-M89 and AITC 117-87).
If bolts carry a tensile load, the washers must provide enough bearing area so that resistance in compression perpindicular to the grain of the wood is not exceeded.
Side plates are frequently used to transfer load from one wood member to another by allowing a butt joint rather than an overlapping joint.
Figure 5.11: Washers for Bolts and Lag Screws