Skyscrapers

The History Of Skyscrapers

The first skyscrapers were constructed in the United States in the 1880s. They arose in urban areas where increased land prices and high population densities created a demand for buildings that rose vertically rather than spread horizontally, thus occupying less precious land area.

By the mid-20^th century, such buildings had become a standard feature of the architectural landscape in most countries in the world.

The development of skyscrapers came as a result of the coincidence of several technological and social developments. The term originally applied to buildings of 10 to 20 storeys, but by the late 20^th century it was used to describe high-rise buildings of unusual height, generally greater than 40 or 50 storeys.

The increase in urban commerce in the United States in the second half of the 19^th century augmented the need for city business space, and the installation of the first safe passenger elevator in 1857 made practical the erection of buildings more than four or five storeys tall. Although the earliest skyscrapers rested on extremely thick masonry walls at the ground level, architects soon turned to use of cast-iron and wrought-iron to support the weight of the upper floors, allowing for more floor space on the lower storeys.

In 1855 William Le Baron Jenney's 10-storey Home Insurance Company Building in Chicago was the first to use steel-girder construction.

As the population density of urban areas has increased, so has the need for buildings that rise rather than spread.

The Empire State Building

It is the steel-framed 102-storeys building completed in New York City in 1931 and located on 5^th Avenue between  33^rd and 34^th streets.

It rises to a height of 1,250 feet ( 381 m ) and was the first skyscraper of such great vertical dimension, so that, when it was completed in 1931, it was the tallest building. Because of its elegant stepped design it is often regarded as the ultimate American skyscrapers.

Characteristics Of Skyscrapers

Structurally, skyscrapers consist of a substructure of piers beneath the ground, a superstructure of columns and girders above the ground, and curtain wall hung on the girders. The foundations of high-rise buildings must sometimes support very heavy gravity loads, and they usually consist of concrete piers, piles, or caissons that that are sunk into the ground. Beds of solid rock are the most desirable base, but ways have been found to describe loads evenly even on relatively  soft ground. The most important factor in the design of high-rise buildings, however, is the building's need to withstand the lateral forces impose by wind and potential earthquakes. Most high-rise have frames made of steel or steel and concrete.

The principal means of vertical transport in a high-rise is the elevator. It is moved by an electric motor that raises or lower the cad in a vertical shaft by means of wire ropes. Because of their height and their large occupant populations, high-rise require the careful provision of life-safety system. Fire-prevention standards should be strict, and provisions for adequate means of egress in case of fire, power failure, or other accident should be provided.

The Highest Buildings In The World

The Council on Tall Buildings and Urban Habit ( Lehigh University, Lehigh, Pennsylvania ) is the international organization that determines the criteria that, in turn, determine the officially accepted heights of buildings. It had the Council's policy that "the height of a building is measured from the sidewalk level to the structural top of a building, including penthouse and tower. Television and radio antennas, mats, and flag poles are not included".

The Sear Tower opened to tenants in 1973, though construction was not actually completed until 1974. Built in Chicago for Sears, Roebuck and Company, the structure reaches 110 floors and height of 1,450 feet ( 442 m ), excluding broadcast antennas and their supports. It was designed by the American engineer Fazlur Khan.

Welded steel frames form vertical tubes that provide the rigidity needed to limit the lateral sway from wind forces. This system minimizes the amount of structural steel required for a building of its great height.

The building is modular in , whit nine 75-square-foot , column-free units.

The building's exterior sheathed in black aluminium and bronze-tinted glass.

The Sears Tower was the world's tallest building until 1996, when it surpassed by the Petronas Twin Tower, a pair of skyscraper office buildings in Kuala Lumpur, Malaysia. The buildings' frames consist of high-strength, steel-reinforced concrete rather than of structural steel.