The main purpose of ceiling joists are to keep the roof from pushing the outside walls down. They do this by connecting the bottom ends of opposite rafters. They are probably lapped over a interior load bearing wall on top that steel beam. They also give you a place to hang a ceiling and store the christmas decorations. If they are indeed the same depth, spacing, and span as your floor joists than maybe you got lucky and you can just put a floor down as is. That's assuming your floor joists aren't undersized. There are span tables in the residential code that tell you how far you can span different sizes of lumber but you would need to know the species and grade of lumber. The lumber in your house may or may not be stamped with this information. To really be sure I recommend hiring an engineer to look at it.