Square Washers are also known as construction square plate washers. Square Washers are typically thin pieces of material with a hole in the middle that is used to distribute the load of a threaded fastener, such as a screw or nut.
It is mostly used instead of the round washers for timber building and concrete floor applications and One of the main advantages of these washers are round washers tend to fold up and split during an earthquake, especially when the sill plate hole is oversized. Square washers, on the other hand, won't have this issue. They can also be used as a wear pad, to reduce vibration, or as a locking device. They are used with all fasteners for larger load distribution and bolting through a wall or timber.