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If your structure has blocks with differing numbers of levels, on levels lower than the topmost level, the program creates floor masses for the entire plan footprint, even within blocks with a roof at that level. It also creates building masses for the roof at the level, resulting in excess mass. This even occurs if the lower level roofs are flat roofs, so that a coinciding floor and roof masses are generated. In this case, it is necessary to go to Load Input view, delete the floor masses created within blocks with a roof at that level. It may alspbe necessary to fine-tune the extents of the building masses to match your structure.You then regenerate seismic loads with the Generate seismic loads only, not building masses box checked.
Refer to Load Generation Process, point 9, for more information about that process.