The definition of a tall building depends first on such considerations as where the building is located. A 20-story building may be considered tall in Evanston, Illinois, but is not considered tall in New York City. It also depends on who is asking the question. For a transportation engineer it is tall when elevator and escalator problems are to be solved; for an architect, when tallness becomes a consideration in planning, aesthetics, and environment; for a fire prevention specialist when the building is taller than the fire-fighting equipment or when the building has to be divided into horizontal fire zones, etc.

For the structural engineer a tall building can be defined as one whose structural system must be modified to make it sufficiently economical to resist lateral forces due to wind or earthquakes within the prescribed criteria for strength, drift, and comfort of the occupants.

A wide utilization of multistory buildings for practical purposes had to wait until many of the technical problems blocking the commercial utilization and the construction of multistory buildings were solved. The most important of the technical developments were efficient and safe vertical elevators and construction materials with a higher strength.

The socio-economic problems that came with the industrialization of the nineteenth century and the insatiable need for space in the cities gave the big impetus to high-rise construction. By the latter part of the?nineteenth century, land in the rapidly developing cities was already so scarce that the only way to go was up.

The first modern multistory building using a steel skeleton was constructed in 1883 in Chicago. It was the ten-story Home Insurance Building utilizing steel I-beams and masonry walls. The big advance in this building was that the weight of the outer shell was not carried on the masonry but was supported on the steel I-beams.

By the turn of the century the downtown business section of New York had established itself as the nation's financial center, and there was a great demand for office space around Wall Street. The first skyscraper at the turn of the century was the 20-story steel skeleton Battery Place Building, World War I slowed down the progress in high-rise construction, and only after the war's conclusion did a new and larger period of high-rise buildings start which continued through the 1920s and into the thirties.

In this period the tall building became a symbol of American development in the eyes of the world, and at home every new skyscraper became a source of amazement and pride.

The crowning glory of this "golden age" of the skyscraper was, obviously, the Empire State Building in New York, which was announced on August 30, 1929. The 102 stories rise to a height of 1 250 ft, and with its modern appendage, the TV antenna, it now has a height of 1 472 ft. The building used 60, 000 tons of steel and 62,000 cubic yards of concrete. Concrete was used for the floor slabs, staircases, and other places where strength, stiffness, and fire protection were needed.

Also, the Rockefeller Center, a new concept of a city within a city, started in this period still during the Depression. The Center contains the towering RCA Building, with its 70 stories, a number of?other high-rise office and commercial buildings, and the world's largest movie house.

The Depression put an end to the great era of skyscrapers, and only in the late 1940s in the wake of World War II did a new era of modern high-rise building begin.

Breaking with tradition, the Secretariat Building of the United Nations completely departed from the lofty towers of the early period, creating a slab-type tower with walls of glass.

As reinforced concrete penetrated the construction field at the turn of the century, its use in high-rise concrete structures also became more widespread. Although prior to World War I high-rise structures were mostly in the domain of structural steel, the foundations and, at times, floor slabs were concrete.

Also, during the post-World War II period, and especially in the fifties, concrete structures developed a personality of their own, reflecting the properties inherent in the material, such as its strength, moldability, fire-safety, handsome appearance, and weather resistance. Contributing to this trend was the widespread use of flat-plate slabs, which utilize two-dimensional load transfer instead of the traditional one-way action. Also, many reinforced concrete buildings of this period abolished the conventional boxlike form and developed?new concepts.

The tallest reinforced concrete building at present is the 74-story?Water Tower Place in Chicago, completed in 1976, with a height of 859 ft. The tower, on top of a 14-story base of commercial space, has a 20-story hotel, and 40 stories of condominiums. The wind resistance of the slender tower is supplied by the peripheral framed tube, which is stiffened by a shear wall connecting its two longitudinal faces.

In the field of steel structures, the late sixties brought the diagonally?braced 100-story John Hancock Building in Chicago; the twin 110-story World Trade Center in New York City; and the crowning glory of the steel high-rise structures, the 120-story Sears Tower in Chicago, which utilizes bundled tubes as the structural system, soaring to a world record of 1450 ft.