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Modelling of different building portions in a single model might be necessary to safeguard against seismic pounding of adjacent building portions, in certain conditions (e.g., different heights, varistion in stiffnesses, insufficient gap) More details, regarding pounding effect in buildings, are available at the following Link.

You will need to try a column having a larger cross-sectional dimensions. It should not be hard to manually calculate the required column size, using the required axial compression capacity, according to the applicable material design code.

Completely different models. There's no point modelling them in same model.

It would be better to include the different portions of the building, separated by expansion joints, in a combined model as well, in order to study the behavior of structural elements located adjacent to expansion joints. For this purpose, the members (columns etc)across the expansion joints may be connected through provision of a strip footing or a raft.

Warnings are clear in details (i.e. type of warning & their locations). You need to correct the problems mentioned in the warnings, through merging or deleting/redrawing the members or elements, one by one. In the essence, there should be no warning message, on using Check Model command. Otherwise, the structure would not be correctly modelled.

Completely different. That means modelling two buildings separately

Check your structure and foundations on all applied loads. You can't skip earthquake if you are designing for it.

1. Because ETABS modifier of 0.25 is different than cracking analysis of SAFE. Are you sure the cracking analysis in SAFE corresponds to 75% cracked section? 2. If you are not performing cracked analysis in SAFE. still results would differ.

Good question/observation. That is the reason codes changed the foundation design from service to ultimate. 1. For your case; make sure there is no tension under service load combinations. Or remove it. 2. Make sure the compression is less than bearing capacity under service load combinations. 3. Once above two steps are done, and before proceeding to ultimate design of reinforcement, make sure that there is zero tension under ultimate load combinations, otherwise the ultimate design will be under-estimated.

Thank you sir for such a detailed report....... i was about to ask you for this. here is a snap shot of my SAP model showing the water pressure on the Tank walls.