Buy Square plate washers online at ParkerTools. Square plate washers are used in conjunction with holding down bolts in a variety of structural applications to secure a steel / timber frame structure to a concrete base. They are most commonly used with a holding down bolt, straight rod, anchor bolt or sometimes loose. The use of plate washers with holding down bolts can provide a larger surface area to the base of the fixing. This mean it would require substantially more force to pull the holding down bolt / anchor out of its embedment.