There is no precise definition of how many stories or what height makes a building a skyscraper. "I don't think it is how many floors you have. I think it is attitude," architect T. J. Gottesdiener told the Christian Science Monitor. Gottesdiener, a partner in the firm of Skidmore, Owings & Merrill, designers of numerous tall buildings including the Sears Tower in Chicago, Illinois, continued, "What is a skyscraper? It is anything that makes you stop, stand, crane your neck back, and look up."
Some observers apply the word "skyscraper" to buildings of at least 20 stories. Others reserve the term for structures of at least 50 stories. But it is widely accepted that a skyscraper fits buildings with 100 or more stories. At 102 stories, the Empire State Building's in New York occupied height reaches 1,224 ft (373 m), and its spire, which is the tapered portion atop a building's roof, rises another 230 ft (70 m). Only 25 buildings around the world stand taller than 1,000 ft (300 m), counting their spires, but not antennas rising above them.
The tallest freestanding structure in the world is the CN Tower in Toronto, Canada, which rises to a height of 1,815 ft (553 m); constructed to support a television antenna, the tower is not designed for human occupation, except for a restaurant and observation deck perched at 1,100 ft (335 m). The world's tallest occupied structure is the Petronas Twin Towers in Kuala Lumpur, Malaysia, which reach a height of 1,483 ft (452 m), including spires. The Sears Tower in Chicago boasts the highest occupied level; the roof of its 110th story stands at 1,453 ft (443 m).
In some ways, super-tall buildings are not practical. It is cheaper to build two half-height buildings than one very tall one. Developers must find tenants for huge amounts of space at one location; for example, the Sears Tower encloses 4.5 million square feet (415,000 square meters). On the other hand, developers in crowded cities must make the fullest possible use of limited amounts of available land. Nonetheless, the decision to build a dramatically tall building is usually based not on economics, but on the desire to attract attention and gain prestige.
Several technological advances occurred in the late nineteenth century that combined to make skyscraper design and construction possible. Among them were the ability to mass produce steel, the invention of safe and efficient elevators, and the development of improved techniques for measuring and analyzing structural loads and stresses. During the 1920s and 1930s, skyscraper development was further spurred by invention of electric arc welding and fluorescent light bulbs (their bright light allowed people to work farther from windows and generated less heat than incandescent bulbs).
Traditionally, the walls of a building supported the structure; the taller the structure, the thicker the walls had to be. A 16-story building constructed in Chicago in 1891 had walls 6 ft (1.8 m) thick at the base. The need for very thick walls was eliminated with the invention of steel-frame construction, in which a rigid steel skeleton supports the building's weight, and the outer walls are merely hung from the frame almost like curtains. The first building to use this design was the 10-story Home Insurance Company Building, which was constructed in Chicago in 1885.
The 792-ft (242-m) tall Woolworth Building, erected in New York City in 1913, first combined all of the components of a true skyscraper. Its steel skeleton rose from a foundation supported on concrete pillars that extended down to bedrock (a layer of solid rock strong enough to support the building), its frame was braced to resist expected wind forces, and its high-speed elevators provided both local and express service to its 60 floors.
In 1931, the Empire State Building rose in New York City like a 1,250-ft (381-m) exclamation point. It would remain the world's tallest office building for 41 years. By 2000, only six other buildings in the world would surpass its height.
Reinforced concrete is one important component of skyscrapers. It consists of concrete (a mixture of water, cement powder, and aggregate consisting of gravel or sand) poured around a gridwork of steel rods (called rebar) that will strengthen the dried concrete against bending motion caused by the wind. Concrete is inherently strong under compressive forces; however, the enormous projected weight of the Petronas Towers led designers to specify a new type of concrete that was more than twice as strong as usual. This high-strength material was achieved by adding very fine particles to the usual concrete ingredients; the increased surface area of these tiny particles produced a stronger bond.
The other primary raw material for skyscraper construction is steel, which is an alloy of iron and carbon. Nearby buildings often limit the amount of space available for construction activity and supply storage, so steel beams of specified sizes and shapes are delivered to the site just as they are needed for placement. Before delivery, the beams are coated with a mixture of plaster and vermiculite (mica that has been heat-expanded to form sponge-like particles) to protect them from corrosion and heat. After each beam is welded into place, the fresh joints are sprayed with the same coating material. An additional layer of insulation, such as fiberglass batting covered with aluminum foil, may then be wrapped around the beams.
To maximize the best qualities of concrete and steel, they are often used together in skyscraper construction. For example, a support column may be formed by pouring concrete around a steel beam.
A variety of materials are used to cover the skyscraper's frame. Known as "cladding," the sheets that form the exterior walls may consist of glass, metals, such as aluminum or stainless steel, or masonry materials, such as granite, marble, or limestone.
Design engineers translate the architect's vision of the building into a detailed plan that will be structurally sound and possible to construct.
Designing a low-rise building involves creating a structure that will support its own weight (called the dead load) and the weight of the people and furniture that it will contain (the live load). For a skyscraper, the sideways force of wind affects the structure more than the weight of the building and its contents. The designer must ensure that the building will not be toppled by a strong wind, and also that it will not sway enough to cause the occupants physical or emotional discomfort.
Each skyscraper design is unique. Major structural elements that may be used alone or in combination include a steel skeleton hidden behind non-load-bearing curtain walls, a reinforced concrete skeleton that is in-filled with cladding panels to form the exterior walls, a central concrete core (open column) large enough to contain elevator shafts and other mechanical components, and an array of support columns around the perimeter of the building that are connected by horizontal beams to one another and to the core.
Because each design is innovative, models of proposed super tall buildings are tested in wind tunnels to determine the effect of high wind on them, and also the effect on surrounding buildings of wind patterns caused by the new building. If tests show the building will sway excessively in strong winds,
In addition to the superstructure, designers must also plan appropriate mechanical systems such as elevators that move people quickly and comfortably, air circulation systems, and plumbing.
Each skyscraper is a unique structure designed to conform to physical constraints imposed by factors like geology and climate, meet the needs of the tenants, and satisfy the aesthetic objectives of the owner and the architect. The construction process for each building is also unique. The following steps give a general idea of the most common construction techniques.
Once construction of a skyscraper is underway, work on several phases of the structure proceeds simultaneously. For example, by the time the support columns are several stories high, workers begin building floors for the lower stories. As the columns reach higher, the flooring crews move to higher stories, as well, and finishing crews begin working on the lowest levels. Overlapping these phases not only makes the most efficient use of time, but it also ensures that the structure remains stable during construction.
The Empire State Building was intended to end the competition for tallest building. It was to tower 102 stories, 1,250 ft (381 m) above Manhattan's streets. Its developers, John J. Raskob and Pierre Samuel Du Pont, along with former New York Governor Alfred E. Smith, announced in August 1929 their intention to build the world's tallest building. They chose the construction firm Starrett Brothers and Eken, and the architectural firm Shreve, Lamb, and Harmon for the project with William F. Lamb as the chief designer. If is set back from the street above the fifth floor and then soars uninterrupted for more than 1,000 ft (305 m) to the 86th floor. The exterior is limestone and granite and vertical chrome-nickel-steel alloy columns extend from the sixth floor to the top. The building contained 67 elevators and 6,500 glass windows, topped with a 200-ft (61-m) mooring mast for dirigibles.
The Empire State Building was completed on April 11, 1931, 12 days ahead of schedule and officially opened on May 1, 1931. The building took its place in history as the tallest building ever built, holding this title for more than 40 years. It was not until 1972, when the 1,348-ft-(411-m-) tall twin towers of the World Trade Center were completed that the Empire State Building was surpassed in height. The World Trade Center in turn was surpassed in 1974 by the Sears Tower in Chicago, which at 1,453 ft (443 mj became the tallest building in the world.
Various factors are taken into consideration when assuring quality control. Because of the huge scale of skyscrapers, a small positioning error at the base will be magnified when extended to the roof. In addition to normal surveying instruments, unusual devices like global positioning system (GPS) sensors and aircraft bombsights may be used to verify the placement and alignment of structural members.
Soil sensors around the building site are used to detect any unexpected earth movement caused by the construction activity.
Excavation of the foundation pit and basement levels require the removal of enormous amounts of dirt. When the 110-story World Trade Center towers were built in New York in the early 1970s, more than I million cubic yards (765,000 cubic meters) of soil and rock were removed and dumped in the Hudson River to create 23.5 acres (95,100 square meters) of new land, on which another skyscraper was later constructed.
Plans have been developed for several new skyscrapers that would break existing height records. For example, a 108-story building at 7 South Dearborn Street in Chicago, expected to be completed by 2004, will be 1,550 ft (473 m) tall. It will provide 43 acres (174,000 square meters) of enclosed space on a lot only 200 ft (61 m) square.
In 1956, American architect Frank Lloyd Wright announced plans for a mile-high (1.6-km tall) skyscraper in which 100,000 people could work. In 1991, another American architect, Dr. Eugene Tsui, designed a 2-mile (3,220-m) tall building that would provide space for living, working, and recreation for 1,000,000 people. Although such buildings may be theoretically constructable, they are currently impractical. For example, human comfort levels limit elevator speeds to no more than 3,000 ft/min (915 m/min). To accommodate the 100,000 people working in Wright's proposed structure, the number of elevator shafts would have taken up too large a portion of the building's area.
Improvements in elevator technology will be important for future skyscraper designs. Self-propelled, cableless elevator cars that move horizontally, as well as vertically, have been proposed, but are still under development. Computerized car dispatching systems using fuzzy logic could be refined to carry people more efficiently by grouping passengers whose destinations are near each other.
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"Ultima's Tower, Two-Mile High Sky City." Tsui Design & Research. http://www.tdrinc.com/ultima.html (March 2000).
— Loretta Hall