When you provide a seismic definition, the program computes horizontal base shear. This base shear is distributed among different stories as storey shears. This storey shears are the applied seismic load on the structure for which you analyses the structure. The load vector is created with this loads. The program soles the structure and finds the displacements and load vector of the static seismic load is wiped out from memory. If you need to reuse the load vectors like you have a REPEAT Load case or SELECT or SELECT OPTIMIZED command, you need to tell the program to keep the load vectors generated for static seismic load case for future use. You need to use PERFORM ANALYSIS command followed by a CHANGE command after each seismic load case.
UBC LOAD X 1
PERFORM ANALYSIS
CHANGE
LOAD 2 LOADTYPE Seismic TITLE UBC LOAD @ Z
UBC LOAD Z 1
PERFORM ANALYSIS
CHANGE
You can simply use SELFWEIGHT Y -1 command to overcome the mentioned problem. Please refer to the attached model.
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