I found these 'girders' in a junk bin at a thrift store, along with numerous Plasticville building parts. They measure 1" x 1" x 2" each. They do not directly connect or interlock with each other. I got a bunch of them, 30 or so, thinking they might be useful for some kind of model project.
It's probably just coincidence that they were mixed in with Plasticville items; I suspect they are from some kind of modular building toy set?