I'm going to be installing a skylight in the middle of this picture, cutting through the middle joist. That means that I'll have to cut through 2 "crossbeams" (I don't think they are called rat runs because I don't have a truss), here is a pic: http://img178.imageshack.us/i/img5813s.jpg/
They are on the other side of my attic as well, 3 of them, 15' each, spanning the entire length of my house, overlapping on a joist as you saw in the first pic.
1. Am I allowed to cut them?
2. If yes, what do I need to do before & after I cut them (i.e. Put additional ones on? Full 15' ones, or just long enough to span a few joists? Move the entire 15' existing ones instead of cutting them?)?
Keep in mind I'll be splaying the skylight front/back so if I need to replace the crossbeams, they will not be in-line with the existing ones, they will be several feet closer to the middle of the attic and/or several feet closer to the edge of the roof to make room for the splayed skylight walls. And yes, I know I need to deal with all those wires that are in the way, fun fun fun.
Got my answer elsewhere. Basically, they are rat runs, not structural in nature (helps folks walk in attic and solidify joists), can just put a couple of additional ones in front & back of the skylight walls spanning 7-8 joists, then I can cut the ones in the picture and double-up everything as needed for the skylight frame. Thanks.