Stair Seismic Design

I've been asked to provide seismic calculations on some mezzanine stairs. I built a model of the stairs in RAM Elements, but have been having some issues trying to confirm that it is running the seismic analysis correctly. Elements seems geared toward using mass for seismic design only when it's in the form of flat rigid diaphragms at discrete levels. I'll have that at the top landing, but it seems to be ignoring the mass of the stair itself. I can use the Self-weight, Earthquake Acceleration, and Response Spectrum tabs to enter much of the information, although I don't see any way to enter Sds. If I use ASCE 7-05 Eq. 13.3-1 for deriving a seismic component force, and try applying that in my 2 orthogonal seismic load  cases, Elements tells me "The load case 'EQx = Seismic in X' already has earthquake. Static loads in nodes are not allowed," and likewise for the Z direction load case.
Does anyone else have any experience with this? Do any of the Bentley people have some insight? I haven't found much help in the help file, and I haven't found this issue addressed in these forums. While this is a small stair, I'd like to develop a good design procedure for use on the larger more complex stairs. Thanks in advance for any suggestions.
Jason