At present, the height of skyscrapers is limited more by elevators than by the strength of steel and concrete! The taller a building is, the more elevators it needs to get people up and down. The point at which ALL the floor space on each floor is occupied by elevator shafts is the absolute limit to the height of a skyscraper. Of course, the PRACTICAL limit is somewhat less because the building needs office space in addition to elevator shafts to be usable as an office building, duh. Also, increasing the speed of elevators improves the ratio of usable floor space to elevator shaft space for a given building. Other "tricks" architects use to push the limit are to use double-decker elevators, and sky-lobbies every (for example) 20 or 30 floors conected by non-stop elevators to the ground floor.