Quite often this kind of arrangement can be seen when beams are connected together. A rectangular plate is welded into a beam, and the plate is bolted to the other beam. I suppose one purpose would be simply that bolting is easier to do in some cases at the site rather than welding the beams together directly at construction site, but is there any other purpose?
I remember once hearing this kind of plate refered to as a moment plate, but I couldn't really find much information on such a thing. Is there some kind of moment-related purpose for this plate or am I just remembering wrong?