To model slabs with variable thickness and loading in SAFE/ETABS, it is best to create a base slab which spans the entire floor, then draw slabs of variable thickness over the base slab to adjust local geometry and loading. Contained-area slabs drawn over the base slab will take precedent. This applies to every area object except drop-panels. To apply variable loading, assign NONE properties to contained-area slabs (aka null areas), then select Assign > Load Data to specify load magnitude and direction. When assigning loads to null areas in both programs, loads will be added to the existing base slab. See notes and table below for additional information:
1- Shell rigid diaphragm must be explicitly assigned to null areas in ETABS for their loads/additional mass to be included in modal analysis (seismic mass).
2- For semi-rigid diaphragms, the contained areas are recommended to be assigned with the same structural properties of the base slab in lieu of null areas. The latter may cause local modes when their load/additional mass are part of the lateral mass source.
3- Refer to the release note below for the enhancement that was recently implemented in ETABS v19.1.0. This new behavior affects ETABS v19.1.0 and later versions, this is in line with the current behavior of SAFE:
"An enhancement was made to make uniform area loads assigned to floor objects with properties to be additive if the objects overlap at any location. This was already true for floor objects with null properties that overlapped objects with properties. Older models where drop panels overlap slab areas and have identical uniform load specifications will have their duplicated loads deleted to maintain the same total loads"