How are you supporting the masonry? Are you directly below it or alongside? If you are along side, the torsion would be too huge for a wide flange at that span.
<1. What deflection limits should I use for the steel beam? Also, should the dead load from the roof joists be considered as live load?>
At that span, deflection is the critical issue and I would consider the total load in the calc and at least meet the l/240 requirement but shoot for l/360 for TL. If there is glass or doors under this opening directly mounted to the bottom, you may even want to consider l/480.
<2. If I design the steel beam as simply supported, is there a potential problem with cracking in the masonry wall due to negative bending stresses?>
Not sure if you meant "design the beam as fixed ends ..." to get negative bending.
If you can dowel into the bottom of the CMU creating sort of a composite type section with the top flange of the new steel beam, that will act as the tension steel at the bottom for a simply supported masonry beam.
Lateral support is your least concern.... worry about the deflection first then attach to the CMU as often as necessary to get the strength necessary in bending.
San Francisco, CA
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