This is a topic that's been on my mind for a while now, and something I think would be an interesting discussion, if not practical. What would it take to convert a parking garage into a residential/commercial/mixed use structure? Has this actually been done anywhere? This is all completely hypothetical of course, as I imagine the costs of retrofitting the building would be higher than new construction on the same site.
First off, I would imagine that a hole would have to be cut in the center for elevator/stairway access, as the exterior elevators would not provide satisfactory access to the building. An HVAC system would also have to be installed, as well as drop ceilings and windows. I would imagine that any other significant structural changes would not need to be made, as the weight of the people, furniture, and walls inside the building would be significantly less than the weight of the cars that it was designed for.