TR.31.8.2 Reference Load Mass Tables
This method uses loads defined in a reference load case as the weights for the response spectrum. You may also define in which global direction the loading is considered for base shear calculation. By default, the program will consider Global Y if no load direction is specified. If no applied loads are found in the Y direction, then it consider the maximum of the total loads in either X or Z directions.
Multiple reference load cases can be used in a single response spectrum, each with a separate load factor applied. This allows you to factor the reference loads for use with weights. For example, if a code-prescribed seismic load requires that 100% of the dead load mass but only 50% of the live load mass be used for seismic loads, then you can use factors of 1.0 for the reference dead load case and 0.5 for the reference live load case.
This has the advantage of allowing you to define a single mass model for use with both static seismic loading as well as response spectra. In order to accomplish this, simply define the response spectrum load case as a reference load and then utilize this reference load in both the static seismic and response spectrum primary load case.