The more open a floor plan appears, where there are not large support columns, the lower the weight bearing capacity will be in general.

With something like modern skyscrapers that appear to have open floor plans, there is a massive structure somewhere within the design. The structure is usually in the center built around the elevators and stairs, although there are other methods too. This structure is engineered for the specific loads of each floor and the total structure, along with various environmental factors such as weather. Like all structures, this starts with a foundation that is large enough for the designed load with a small margin of safety added. The cost of the foundation is directly related to the weight bearing capacity. No one is building structures with substantial extra unused capacity. Likewise, people like the aesthetics of open indoor spaces. This involves designing a structure with the minimal amount of load bearing capacity so that it does not need support columns throughout for the roof and upper floors. This particular aesthetic constraint means that the total load bearing capacity of department stores is very close to what you see in a typical store. If you start adding a bunch of walls and furniture to subdivide this space, there us absolutely no chance that the structure could handle the load. Even if you would like to add support columns, the foundation is engineered for the load. You can't reinforce something like this in a cost effective way. The size and depth of the pad, rebar density and structure all must be substantially different. You're likely to need piles or other features that tie in the structure to deeper bedrock elements of the underlying earth.