Some buildings are called skyscrapers because they are of great height and have an iron or steel frame inside that supports its floors and walls. Before builders figured out how to make such frames, stone or brick walls had to bear the weight of structures, which could not stand up if they were made too high. And tall stone or brick buildings had to have very thick walls on lower floors to bear the weight of the walls and floors above them. These thick walls wasted a lot of useful space. Because cities have limited land, builders experimented with materials and construction methods in an effort to construct taller buildings that were more practical. Finally, in 1885, William Le Baron Jenny built the first modern skyscraper in Chicago. While just 10 stories high, which seems short by today’s standards, the Home Insurance Company Building was the first structure to have an internal steel skeleton bear all of its weight. From that point on, tall buildings began to soar into the air, scraping the sky. Just 30 years after the first skyscraper was built, buildings were erected that reached 60 stories high.
The first elevators in use were not especially safe because once in a while a cable would break, and a car, pulled by gravity, would come crashing down. Safety devices were soon added, though, to keep such disasters from occurring. (American inventor Elisha Otis invented the first “safety” elevator in 1853.) Additional ropes attached to cars and powerful metal “jaws” that grip guard rails keep elevators from falling if their main cables break. Other safety devices keep elevators from moving when their doors are still open and from traveling too fast. Automatic switches in the shaft allow an elevator to hurry past unwanted floors, or to slow and stop when a chosen floor is reached, unlocking its doors to admit and release passengers. Very long elevators are not always practical, so some buildings use one set of elevators to take passengers part way up the building and another set to service the upper floors.