Here is an example of when the IRC calls for hangers at ceiling joists:

From 802.9, IRC 2003

"Approved hangers shall be used for the header joist to trimmer joist connections when the header joist span exceeds 6 feet (1829 mm). Tail joists over 12 feet (3658 mm) long shall be supported at the header by framing anchors or on ledger strips not less than 2 inches by 2 inches(51 mm by 51 mm)."

Here is another one in the case where the 2nd story floor is also the 1st story ceiling I think you could call them floor or ceiling joists:

502.6 Bearing.
The ends of each joist, beam or girder shall have not less than 1.5 inches (38 mm) of bearing on wood or metal and not less than 3 inches (76 mm) on masonry or concrete except where supported on a 1-inch-by-4-inch (25.4 mm by 102 mm) ribbon strip and nailed to the adjacent stud or by the use of approved joist hangers.

Eric