A girder coupler is a type of mechanical fastener used to connect two girders together. It is typically used on construction sites to allow workers to construct large structures from multiple girders. A girder coupler typically consists of two metal plates connected with a pin or a threaded rod. The plates are designed to fit over the flanges of two girders and the pin or rod is used to hold the plates together. In some cases, a girder coupler may also include a locking mechanism to help ensure the connection is secure.