I modeled a 3 stringer bridge with plates. I created offsets for the beams, but immediately after doing that dashed lines extending out appeared. Could anyone explain what's going on here?

I am modeling a very wide slab (120' x 30') simply supported on two long opposite edges. It would have been a simple one way slab but it has two edge beams on the shorter (30') edges. The slab is 20" deep and the edge beams are 29" deep x 18" wide. I…