the centers of most cities became business areas after the invention of what
Downtown The skyscraper would become the hallmark of the downtown area. Prior to the invention of the elevator – and later the high-speed elevator – buildings were limited in height to about six stories, which was a de facto limit set by the amount of stairs it was assumed that people would climb, but with the elevator, that limit was shattered, and buildings began to be constructed up to about sixteen stories. What limited them then was the thickness of the masonry needed at the base to hold the weight of the building above it. As the buildings got taller, the thickness of the masonry and the space needed for elevators did not allow for sufficient rentable space to make the building profitable. What shattered that restriction was the invention of first the iron- and then the steel frame building, in which the building's load was carried by an internal metal frame skeleton, which the masonry – and later glass – simply hung off of without carrying any weight.[7]