That's probably one of the easiest forms of load to analyze, since pressure is usually the most uniformly distributed load you can have, there should be no torsional load created on the channel. Channels do not perform especially well resisting torsional loads.
Just use the normal shear and bending moment calculation formula appropriate for your channel's end boundary conditions to calculate shear stress at the supports and bending stress at center span.
Use the 2001 AISI NASPEC for weak axis bending of cold-formed steel chanels. Though lateral-torsional buckling is not usually an issue with weak-axis bending, a channel will experience distortional buckling depending on its orientation relative to the line of force. The AISI code should cover this situation.
