Skyscraper

For other uses, see Skyscraper (disambiguation).

Skyscrapers in Chicago, United States. The Willis Tower (designed by Khan and Graham) in the middle has 108 storeys and is 442 metres (1,450 feet) tall. It utilizes the bundled tube design.

A skyscraper is a tall, continuously habitable building of many storeys, usually designed for office and commercial use. There is no official definition or height above which a building may be classified as a skyscraper. One common feature of skyscrapers is having a steel framework from which curtain walls are suspended, rather than load-bearing walls of conventional construction. Most skyscrapers have a steel frame that enables the construction of load-bearing walls taller than of those made of reinforced concrete. Skyscrapers' walls are not load-bearing, and therefore most skyscrapers are characterized by large surface areas of windows made possible by the concept of steel frame and curtain walls. However, skyscrapers can have curtain walls that mimic conventional walls and a small surface area of windows.

Skyscrapers since 1960s utilize the tubular designs, innovated by a Bangladeshi-American structural engineer named Fazlur Rahman Khan. This engineering principle makes the buildings structurally more efficient and stronger. It reduces the usage of material (economically much more efficient), while simultaneously allows the buildings to reach greater heights. It allows fewer interior columns, and so creates more usable floor space. It further enables buildings to take on various shapes. There are several variations of the tubular design; these structural systems are fundamental to tall building design today. After the great depression, skyscraper construction was abandoned. Bangladeshi-American structural engineer Fazlur Khan, more than any others, ushered in a renaissance in skyscrapers construction from 1960s with structural innovations that transformed the industry. and made it possible for people to live and work in "cities in the sky". Other pioneers include Hal Iyengar, William Lemesserier, and etc. Cities have experienced a huge surge in skyscraper construction. Fazlur Rahman Khan is regarded as the "Einstein of Structural Engineering" for his revolutionary work which remain fundamental to modern skyscraper construction. Khan created a legacy of innovations that is unparalleled and became an icon in both architecture and structural engineering.

Today, skyscrapers are an increasingly common sight where land is expensive, as in the centres of big cities, because they provide such a high ratio of rentable floor space per unit area of land. They are built not just for economy of space; like temples and palaces of the past, skyscrapers are considered symbols of a city's economic power. Not only do they define the skyline, they help to define the city's identity as easily recognizable landmarks. In some cases exceptionally tall skyscrapers have been built, not out of necessity, but to help define the city's identity and presence or power as a city.

Definition

A relatively small building may be considered a skyscraper if it protrudes well above its built environment and changes the overall skyline. The maximum height of structures has progressed historically with building methods and technologies and thus what we today consider a skyscraper is taller than before. Also lacking an official definition, the term 'supertall' has arisen for the current generation of exceptionally tall buildings. High-rise buildings are considered shorter than skyscrapers. Home Insurance Building in Chicago, United States was considered a skyscraper when it was built in 1884, but it had only ten storeys. Today such a building would not be considered a skyscraper. There is no clear definition of any difference between a tower block and a skyscraper though a building lower than about thirty storeys is not likely to be a skyscraper and a building with fifty or more storeys is certainly a skyscraper.

The term "skyscraper" was first applied to buildings of steel framed construction of at least 10 storeys in the late 19th century, a result of public amazement at the tall buildings being built in major cities like Chicago, New York City, Detroit, and St. Louis. The first steel frame skyscraper was the Home Insurance Building (originally 10 storeys with a height of 42 m or 138 ft) in Chicago, Illinois in 1885. Some point to New York's seven-floor Equitable Life Assurance Building, built in 1870, as an early skyscraper for its innovative use of a kind of skeletal frame, but such designation depends largely on what factors are chosen. Even the scholars making the argument find it to be purely academic.

The structural definition of the word skyscraper was refined later by architectural historians, based on engineering developments of the 1880s that had enabled construction of tall multi-storey buildings. This definition was based on the steel skeleton—as opposed to constructions of load-bearing masonry, which passed their practical limit in 1891 with Chicago's Monadnock Building.

What is the chief characteristic of the tall office building? It is lofty. It must be tall. The force and power of altitude must be in it, the glory and pride of exaltation must be in it. It must be every inch a proud and soaring thing, rising in sheer exaltation that from bottom to top it is a unit without a single dissenting line.

—Louis Sullivan's The Tall Office Building Artistically Considered (1896)

The steel frame developed in stages of increasing self-sufficiency, with several buildings in Chicago and New York advancing the technology that allowed the steel frame to carry a building on its own. Today, however, many of the tallest skyscrapers are built almost entirely with reinforced concrete.

The Emporis Standards Committee defines a high-rise building as "a multi-storey structure between 35–100 meters tall, or a building of unknown height from 12–39 floors" and a skyscraper as "a multi-storey building whose architectural height is at least 100 m or 330 ft." Some structural engineers define a highrise as any vertical construction for which wind is a more significant load factor than earthquake or weight. Note that this criterion fits not only high-rises but some other tall structures, such as towers.

The word skyscraper often carries a connotation of pride and achievement. The skyscraper, in name and social function, is a modern expression of the age-old symbol of the world center or axis mundi: a pillar that connects earth to heaven and the four compass directions to one another.

A loose convention of some in the United States and Europe draws the lower limit of a skyscraper at 150 m or 490 ft.

The term 'supertall' has recently been coined.

The Council on Tall Buildings and Urban Habitat (CTBUH) defines “supertall” as a building over 300 m (980 ft) in height. Although great heights are now being achieved with built tall buildings—in excess of 800 m (2,600 ft)—at the mid-point of 2011 there [were] only approximately 54 buildings in excess of 300 m (980 ft) completed and occupied globally.

— CTBUH

History

The Two Towers of Bologna in the 12th century reached 97.2 m (319 ft) in height.

The 16th-century city of Shibam consisted entirely of over 500 high-rise tower houses.

Pre-19th century

Modern skyscrapers are built with steel or reinforced concrete frameworks and curtain walls of glass or polished stone. They utilize mechanical equipment such as water pumps and elevators. Until the 19th century, buildings of over six storeys were rare, as having great numbers of stairs to climb was impractical for inhabitants, and water pressure was usually insufficient to supply running water above 50 m (164 ft).

The tallest building in ancient times was the 146 m (479 ft) Great Pyramid of Giza in ancient Egypt, built in the 26th century BCE. It was not surpassed in height for thousands of years, the 14th century CE Lincoln Cathedral being conjectured by many to exceed it. The latter in turn was not surpassed until the 555 feet (169 m) Washington Monument in 1884. However, being uninhabited, none of these structures actually complies with the modern definition of a skyscraper.

High-rise apartments flourished in classical antiquity. Ancient Roman insulae there and in other imperial cities reached 10 and more storeys. Beginning with Augustus (r. 30 BCE-14 CE), several emperors attempted to establish limits of 20–25 m for multi-storey buildings, but met with only limited success. Lower floors were typically occupied by shops or wealthy families, the upper rented to the lower classes. Surviving Oxyrhynchus Papyri indicate that seven-storey buildings existed in provincial towns such as in 3rd century CE Hermopolis in Roman Egypt.

The skylines of many important medieval cities had large numbers of high-rise urban towers, built by the wealthy for defense and status. The residential Towers of 12th century Bologna numbered between 80 to 100 at a time, the tallest of which is the 97.2 m (319 ft) high Asinelli Tower. A Florentine law of 1251 decreed that all urban buildings be immediately reduced to less than 26 m. Even medium-sized towns of the era are known to have proliferations of towers, such as the 72 up to 51 m height in San Gimignano.

The medieval Egyptian city of Fustat housed many high-rise residential buildings, which Al-Muqaddasi in the 10th century described as resembling minarets. Nasir Khusraw in the early 11th century described some of them rising up to 14 storeys, with roof gardens on the top floor complete with ox-drawn water wheels for irrigating them. Cairo in the 16th century had high-rise apartment buildings where the two lower floors were for commercial and storage purposes and the multiple storeys above them were rented out to tenants. An early example of a city consisting entirely of high-rise housing is the 16th-century city of Shibam in Yemen. Shibam was made up of over 500 tower houses, each one rising 5 to 11 storeys high, with each floor being an apartment occupied by a single family. The city was built in this way in order to protect it from Bedouin attacks. Shibam still has the tallest mudbrick buildings in the world, with many of them over 30 m (98 ft) high.

An early modern example of high-rise housing was in 17th-century Edinburgh, Scotland, where a defensive city wall defined the boundaries of the city. Due to the restricted land area available for development, the houses increased in height instead. Buildings of 11 storeys were common, and there are records of buildings as high as 14 storeys. Many of the stone-built structures can still be seen today in the old town of Edinburgh. The oldest iron framed building in the world, although only partially iron framed, is The Flaxmill (also locally known as the "Maltings"), in Shrewsbury, England. Built in 1797, it is seen as the "grandfather of skyscrapers”, since its fireproof combination of cast iron columns and cast iron beams developed into the modern steel frame that made modern skyscrapers possible. Unfortunately, it lies derelict and needs much investment to keep it standing.

Oriel Chambers, Liverpool. The world's first glass curtain walled building. The stone mullions are decorative.

The Wainwright Building, a 10-storey red brick office building in St. Louis, Missouri, built in 1891

Early skyscrapers

In 1852 Elisha Otis introduced the safety elevator, allowing convenient and safe passenger movement to upper floors. Another crucial development was the use of a steel frame instead of stone or brick, otherwise the walls on the lower floors on a tall building would be too thick to be practical. An early development in this area was Oriel Chambers in Liverpool. Designed by local architect Peter Ellis in 1864, the building was the world's first iron-framed, glass curtain-walled office building. It was only 5 floors high. Further developments led to the world's first skyscraper, the ten-storey Home Insurance Building in Chicago, built in 1884–1885. While its height is not considered very impressive today, it was at that time. The architect, Major William Le Baron Jenney, created a load-bearing structural frame. In this building, a steel frame supported the entire weight of the walls, instead of load-bearing walls carrying the weight of the building. This development led to the "Chicago skeleton" form of construction.

Burnham and Root's 1889 Rand McNally Building in Chicago, 1889, was the first all-steel framed skyscraper, while Louis Sullivan's Wainwright Building in St. Louis, Missouri, 1891, was the first steel-framed building with soaring vertical bands to emphasize the height of the building and is therefore considered by some to be the first true skyscraper.

Most early skyscrapers emerged in the land-strapped areas of Chicago, London, and New York toward the end of the 19th century. A land boom in Melbourne, Australia between 1888–1891 spurred the creation of a significant number of early skyscrapers, though none of these were steel reinforced and few remain today. Height limits and fire restrictions were later introduced. London builders soon found building heights limited due to a complaint from Queen Victoria, rules that continued to exist with few exceptions until the 1950s. Concerns about aesthetics and fire safety had likewise hampered the development of skyscrapers across continental Europe for the first half of the twentieth century (with the notable exceptions of the 1898 Witte Huis (White House) in Rotterdam; the Royal Liver Building in Liverpool, completed in 1911 and 90 m (300 ft) high; and the 17-storey Kungstornen (Kings' Towers) in Stockholm, Sweden, which were built 1924–25, the 15-storey Edificio Telefónica in Madrid, Spain, built in 1929; the 26-storey Boerentoren in Antwerp, Belgium, built in 1932; and the 31-storey Torre Piacentini in Genoa, Italy, built in 1940). After an early competition between Chicago and New York City for the world's tallest building, New York took the lead by 1895 with the completion of the American Surety Building, leaving New York with the title of tallest building for many years. New York City developers competed among themselves, with successively taller buildings claiming the title of "world's tallest" in the 1920s and early 1930s, culminating with the completion of the Chrysler Building in 1930 and the Empire State Building in 1931, the world's tallest building for forty years. The first completed World Trade Center tower became the world's tallest building in 1972. However, it was soon overtaken by the Sears Tower (now Willis Tower) in Chicago within two years. The Sears Tower stood as the world's tallest building for 24 years, from 1974 until 1998, until it was edged out by Petronas Twin Towers in Kuala Lumpur, which held the title for six years.

Modern skyscrapers

From the 1930s onwards, skyscrapers also began to appear in Latin America (São Paulo, Rio de Janeiro, Buenos Aires, Santiago, Caracas, Bogotá, Mexico City) and in Asia (Tokyo, Shanghai, Hong Kong, Manila, Singapore, Mumbai, Seoul, Kuala Lumpur, Taipei, Bangkok). Immediately after World War II, the Soviet Union planned eight massive skyscrapers dubbed "Stalin Towers" for Moscow; seven of these were eventually built. The rest of Europe also slowly began to permit skyscrapers, starting with Madrid, during the 1950s. Finally, skyscrapers also began to be constructed in cities of Africa, the Middle East and Oceania (mainly Australia) from the late 1950s. After the Great depression skyscrapers construction was abandoned for over thirty years. Fazlur Khan, more than any others, ushered in a renaissance in skyscrapers construction from 1960s with structural innovations that transformed the industry. Other pioneers include Hal Iyengar, William Lemesserier, and etc.

In the early 1960s structural engineer Fazlur Khan realized that the rigid steel frame structure that had dominated tall building design and construction so long was not the only system fitting for tall buildings, marking the beginning of a new era of skyscraper revolution in terms of multiple structural systems. His central innovation in skyscraper design and construction was the idea of the "tube" structural system, including the "framed tube", "trussed tube", and "bundled tube". These systems allowed far greater economic efficiency, and also allowed efficient skyscrapers to take on various shapes, no longer needing to be box-shaped. Over the next fifteen years, many towers were built by Khan and the "Second Chicago School", including the massive 442 m (1,450 ft) Willis Tower. The tubular systems are fundamental to tall building design. Most buildings over 40-storeys constructed since the 1960s now use a tube design derived from Khan’s structural engineering principles. Since 2000, Cities like Chicago, Shanghai, Dubai, New York, and Toronto have experienced a huge surge in skyscraper construction, thanks to Khan's innovations allowing economic skyscrapers. Chicago, Hong Kong, and New York City, otherwise known as "the big three," are recognized in architectural circles as having especially compelling skylines. A landmark skyscraper can inspire a boom of new high-rise projects in its city, as Taipei 101 has done in Taipei since its opening in 2004. In 2010, The Bank of America Tower at One Bryant Park became the world's first commercial LEED Platinum skyscraper.

Design and construction

Main article: Skyscraper design and construction

The design and construction of skyscrapers involves creating safe, habitable spaces in very tall buildings. The buildings must support their weight, resist wind and earthquakes, and protect occupants from fire. Yet they must also be conveniently accessible, even on the upper floors, and provide utilities and a comfortable climate for the occupants. The problems posed in skyscraper design are considered among the most complex encountered given the balances required between economics, engineering, and construction management.

One common feature of skyscrapers is having a steel framework from which curtain walls are suspended, rather than load-bearing walls of conventional construction. Most skyscrapers have a steel frame that enables to build taller than load-bearing walls of reinforced concrete. Skyscrapers usually have particularly small surface area of what are conventionally thought of as walls, because the walls are not load-bearing and therefore most skyscrapers are characterized by large surface areas of windows made possible by the concept of steel frame and curtain walls. However, skyscrapers can have curtain walls that mimick conventional walls and a small surface area of windows.

The concept of a skyscraper is a product of the industrialized age, made possible by cheap fossil fuel derived energy and industrially refined raw materials such as steel and concrete. The construction of skyscrapers was enabled by steel frame construction that surpassed brick and mortar construction starting at the end of the 19th century and finally surpassing it in the 20th century together with reinforced concrete construction as the price of steel decreased and labour costs increased.

The steel frames become inefficient and uneconomic for supertall buildings as usable floor spaces are reduced for supporting column and due to more usage of steel. Since 1960 Tubular designs are used for high rises. This conception reduces the usage of material (more efficient in economic terms - Willis Tower uses 2/3 of the steel as Empire state building), yet allows greater height. It allows fewer interior columns, and so create more usable floor space. It further enables buildings to take on various shapes.

The amount of steel, concrete and glass needed to construct a single skyscraper is large, and these materials represent a great deal of embodied energy. Skyscrapers are thus energy intensive buildings, but skyscrapers have a long lifespan, for example the Empire State Building in New York City, United States completed in 1931 and is still in active use. Skyscrapers have considerable mass, which means that they must be built on a sturdier foundation than would be required for shorter, lighter buildings. Building materials must also be lifted to the top of a skyscraper during construction, requiring more energy than would be necessary at lower heights. Furthermore, a skyscraper consumes a lot of electricity because potable and non-potable water have to be pumped to the highest occupied floors, skyscrapers are usually designed to be mechanically ventilated, elevators are generally used instead of stairs, and natural lighting cannot be utilized in rooms far from the windows and the windowless spaces such as elevators, bathrooms and stairwells.

Elevators are characteristic to skyscrapers. In 1852 Elisha Otis introduced the safety elevator, allowing convenient and safe passenger movement to upper floors. Another crucial development was the use of a steel frame instead of stone or brick, otherwise the walls on the lower floors on a tall building would be too thick to be practical. Today major manufacturers of elevators include Otis, ThyssenKrupp, Schindler, and KONE. For supertall buildings sky lobbies with express elevators invented by F.R Khan are used.

Khan realized that the rigid steel frame structure that had "dominated tall building design and construction so long was not the only system fitting for tall buildings", marking "the beginning of a new era of skyscraper revolution in terms of multiple structural systems." Khan's design innovations significantly improved the construction of high-rise buildings, enabling them to withstand enormous forces generated on these super structures. These new designs opened an economic door for contractors, engineers, architects, and investors, providing vast amounts of real estate space on minimal plots of land.

Basic design considerations

Good structural design is important in most building design, but particularly for skyscrapers since even a small chance of catastrophic failure is unacceptable given the high price. This presents a paradox to civil engineers: the only way to assure a lack of failure is to test for all modes of failure, in both the laboratory and the real world. But the only way to know of all modes of failure is to learn from previous failures. Thus, no engineer can be absolutely sure that a given structure will resist all loadings that could cause failure, but can only have large enough margins of safety such that a failure is acceptably unlikely. When buildings do fail, engineers question whether the failure was due to some lack of foresight or due to some unknowable factor.

Loading and vibration

Taipei 101 endures a typhoon (2005)

The load a skyscraper experiences is largely from the force of the building material itself. In most building designs, the weight of the structure is much larger than the weight of the material that it will support beyond its own weight. In technical terms, the dead load, the load of the structure, is larger than the live load, the weight of things in the structure (people, furniture, vehicles, etc.). As such, the amount of structural material required within the lower levels of a skyscraper will be much larger than the material required within higher levels. This is not always visually apparent. The Empire State Building's setbacks are actually a result of the building code at the time, and were not structurally required. On the other hand John Hancock Center's shape is uniquely the result of how it supports loads. Vertical supports can come in several types, among which the most common for skyscrapers can be categorized as steel frames, concrete cores, tube within tube design, and shear walls.

The wind loading on a skyscraper is also considerable. In fact, the lateral wind load imposed on super-tall structures is generally the governing factor in the structural design. Wind pressure increases with height, so for very tall buildings, the loads associated with wind are larger than dead or live loads.

Other vertical and horizontal loading factors come from varied, unpredictable sources, such as earthquakes.

Shear walls

A shear wall, in its simplest definition, is a wall where the entire material of the wall is employed in the resistance of both horizontal and vertical loads. A typical example is a brick or cinderblock wall. Since the wall material is used to hold the weight, as the wall expands in size, it must hold considerably more weight. Due to the features of a shear wall, it is acceptable for small constructions, such as suburban housing or an urban brownstone, to require low material costs and little maintenance. In this way, shear walls, typically in the form of plywood and framing, brick, or cinderblock, are used for these structures. For skyscrapers, though, as the size of the structure increases, so does the size of the supporting wall. Large structures such as castles and cathedrals inherently addressed these issues due to a large wall being advantageous (castles), or ingeniously designed around (cathedrals). Since skyscrapers seek to maximize the floor-space by consolidating structural support, shear walls tend to be used only in conjunction with other support systems.

Steel frame

The classic concept of a skyscraper is a large steel box with many small boxes inside it. The genius of the steel frame is its simplicity. By eliminating the inefficient part of a shear wall, the central portion, and consolidating support members in a much stronger material, steel, a skyscraper could be built with both horizontal and vertical supports throughout. This method, though simple, has drawbacks. Chief among these is that as more material must be supported (as height increases), the distance between supporting members must decrease, which actually in turn, increases the amount of material that must be supported. This becomes inefficient and uneconomic for buildings above 40 stories tall as usable floor spaces are reduced for supporting column and due to more usage of steel.

Tube structural systems

See also: Tube (structure)

The Willis Tower showing the bundled tube frame design

Since 1963, a new structural system of framed tubes appeared. Fazlur Khan and J. Rankine defined the framed tube structure as "a three dimensional space structure composed of three, four, or possibly more frames, braced frames, or shear walls, joined at or near their edges to form a vertical tube-like structural system capable of resisting lateral forces in any direction by cantilevering from the foundation." Closely spaced interconnected exterior columns form the tube. Horizontal loads (primarily wind) are supported by the structure as a whole. About half the exterior surface is available for windows. Framed tubes allow fewer interior columns, and so create more usable floor space. Where larger openings like garage doors are required, the tube frame must be interrupted, with transfer girders used to maintain structural integrity. Tube structures cut down costs, at the same time allow buildings to reach greater heights. Tube-frame construction was first used in the DeWitt-Chestnut Apartment Building, designed by Khan and completed in Chicago in 1963. It was used soon after for the John Hancock Center and in the construction of the World Trade Center.

A variation on the tube frame is the bundled tube, which uses several interconnected tube frames. The Willis Tower in Chicago used this design, employing nine tubes of varying height to achieve its distinct appearance. The bundle tube design was not only highly efficient in economic terms, but it was also "innovative in its potential for versatile formulation of architectural space. Efficient towers no longer had to be box-like; the tube-units could take on various shapes and could be bundled together in different sorts of groupings." The bundled tube structure meant that "buildings no longer need be boxlike in appearance: they could become sculpture."

The tubular systems are fundamental to tall building design. Most buildings over 40-storeys constructed since the 1960s now use a tube design derived from Khan’s structural engineering principles, examples including the construction of the World Trade Center, Aon Centre, Petronas Towers, Jin Mao Building, and most other supertall skyscrapers since the 1960s. The strong influence of tube structure design is also evident in the construction of the current tallest skyscraper, the Burj Khalifa.

Fazlur Rahman Khan, the father of tubular design, is the most influential structural engineer of the 20th century. He has been called the "Einstein of Structural Engineering" for his revolutionary work which remain fundamental to modern skyscraper construction. His breakthroughs in structural engineering for tall and long-span buildings exerted an unprecedented and lasting influence on the profession, both nationally and internationally. With a career marked by a legacy of innovations that is unpeered, Khan has become an icon in both architecture and structural engineering.

Changes of structure with height. The tubular systems by F.R Khan are fundamental for super tall buildings.

Khan's central innovation in skyscraper design and construction was the idea of the "tube" structural system for tall buildings, including the "framed tube", "trussed tube" and "bundled tube" variations. His "tube concept," using "all the exterior wall perimeter structure of a building to simulate a thin-walled tube, revolutionized tall building design." The constructions of most supertall skyscrapers since the 1960s, including the construction of the World Trade Center, Petronas Towers and Jin Mao Building, employ a tube structural system.

Framed tube

Since 1963, the new structural system of framed tubes became highly influential in skyscraper design and construction. Khan defined the framed tube structure as "a three dimensional space structure composed of three, four, or possibly more frames, braced frames, or shear walls, joined at or near their edges to form a vertical tube-like structural system capable of resisting lateral forces in any direction by cantilevering from the foundation." Closely spaced interconnected exterior columns form the tube. Horizontal loads, for example from wind and earthquakes, are supported by the structure as a whole. About half the exterior surface is available for windows. Framed tubes allow fewer interior columns, and so create more usable floor space. The bundled tube structure is more efficient for tall buildings, lessening the penalty for height. The structural system also allows the interior columns to be smaller and the core of the building to be free of braced frames or shear walls that use up valuable floor space. Where larger openings like garage doors are required, the tube frame must be interrupted, with transfer girders used to maintain structural integrity.

The first building to apply the tube-frame construction was the DeWitt-Chestnut Apartments building that Khan designed and was completed in Chicago in 1963. This laid the foundations for the framed tube structure used in the construction of the World Trade Center.

The John Hancock Center, designed by Skidmore, Owings and Merrill with chief designer Bruce Graham and structural engineer Fazlur Khan. The building was completed in 1969.

Trussed tube and X-bracing

Khan pioneered several other variations of the tube structure design. One of these was the concept of X-bracing, or the "trussed tube", first employed for the John Hancock Center. This concept reduced the lateral load on the building by transferring the load into the exterior columns. This allows for a reduced need for interior columns thus creating more floor space. This concept can be seen in the John Hancock Center, designed in 1965 and completed in 1969. One of the most famous buildings of the structural expressionist style, the skyscraper's distinctive X-bracing exterior is actually a hint that the structure's skin is indeed part of its 'tubular system'. This idea is one of the architectural techniques the building used to climb to record heights (the tubular system is essentially the spine that helps the building stand upright during wind and earthquake loads). This X-bracing allows for both higher performance from tall structures and the ability to open up the inside floorplan (and usable floor space) if the architect desires. Original features such as the skin, pioneered by Fazlur Khan, have made the John Hancock Center an architectural icon.

In contrast to earlier steel-frame structures, such as the Empire State Building (1931), which required about 206 kilograms of steel per square metre and Chase Manhattan Bank Building (1961), which required around 275 kilograms of steel per square metre, the John Hancock Center was far more efficient, requiring only 145 kilograms of steel per square metre. The trussed tube concept was applied to many later skyscrapers, including the Onterie Center, Citigroup Center and Bank of China Tower.

Sears Tower (now Willis Tower), designed by Bruce Graham and Fazlur Khan and completed in 1974, was the tallest building in the world at the time of its construction

Bundle tube

One of Khan's most important variations of the tube structure concept was the "bundled tube," which he used for the Sears Tower and One Magnificent Mile. The bundle tube design was not only the most efficient in economic terms, but it was also "innovative in its potential for versatile formulation of architectural space. Efficient towers no longer had to be box-like; the tube-units could take on various shapes and could be bundled together in different sorts of groupings."

Concrete tube structures

The last major buildings engineered by Khan were the One Magnificent Mile and Onterie Center in Chicago, which employed his bundled tube and trussed tube system designs respectively. In contrast to his earlier buildings, which were mainly steel, his last two buildings were concrete. His earlier DeWitt-Chestnut Apartments building, built in 1963 in Chicago, was also a concrete building with a tube structure.

The influence of Khan's tube structure design can be seen in numerous buildings built since the 1960s. Tube structures have since been used in many skyscrapers, including the construction of the World Trade Center, Petronas Towers, Jin Mao Building, and most other supertall skyscrapers since the 1960s. The strong influence of tube structure design is also evident in the world's current tallest skyscraper, the Burj Khalifa in Dubai. According to Stephen Bayley of The Daily Telegraph:

Khan invented a new way of building tall. [...] So Fazlur Khan created the unconventional skyscraper. Reversing the logic of the steel frame, he decided that the building's external envelope could – given enough trussing, framing and bracing – be the structure itself. This made buildings even lighter. The "bundled tube" meant buildings no longer need be boxlike in appearance: they could become sculpture. Khan's amazing insight – he was name-checked by Obama in his Cairo University speech last year – changed both the economics and the morphology of supertall buildings. And it made Burj Khalifa possible: proportionately, Burj employs perhaps half the steel that conservatively supports the Empire State Building. [...] Burj Khalifa is the ultimate expression of his audacious, lightweight design philosophy.

The elevator conundrum

Elevators in the Empire State Building

The invention of the elevator was a precondition for the invention of skyscrapers, given that most people would not (or could not) climb more than a few flights of stairs at a time. The elevators in a skyscraper are not simply a necessary utility, like running water and electricity, but are in fact closely related to the design of the whole structure: a taller building requires more elevators to service the additional floors, but the elevator shafts consume valuable floor space. If the service core, which contains the elevator shafts, becomes too big, it can reduce the profitability of the building. Architects must therefore balance the value gained by adding height against the value lost to the expanding service core. Many tall buildings use elevators in a non-standard configuration to reduce their footprint. Buildings such as the former World Trade Center Towers and Chicago's John Hancock Center use sky lobbies, where express elevators take passengers to upper floors which serve as the base for local elevators. This allows architects and engineers to place elevator shafts on top of each other, saving space. Sky lobbies and express elevators take up a significant amount of space, however, and add to the amount of time spent commuting between floors. Other buildings, such as the Petronas Towers, use double-deck elevators, allowing more people to fit in a single elevator, and reaching two floors at every stop. It is possible to use even more than two levels on an elevator, although this has never been done. The main problem with double-deck elevators is that they cause everyone in the elevator to stop when only people on one level need to get off at a given floor.

Sky lobby

The first sky lobby was also designed by Khan for the John Hancock Center. Later buildings with sky lobbies include the World Trade Center, Petronas Twin Towers and Taipei 101. The 44th-floor sky lobby of the John Hancock Center also features the first high-rise indoor swimming pool, which remains the highest in America. This was the first time that people could have the opportunity to work and live "in the sky".

Economic rationale

Skyscrapers are usually situated in city centers where the price of land is high. Constructing a skyscraper becomes justified if the price of land is so high that it makes economic sense to build upwards as to minimize the cost of the land per the total floor area of a building. Thus the construction of skyscrapers is dictated by economics and results in skyscrapers in a certain part of a large city unless a building code restricts the height of buildings. Skyscrapers are rarely seen in small cities and they are characteristic of large cities, because of the critical importance of high land prices for the construction of skyscrapers. Usually only office, commercial and hotel users can afford the rents in the city center and thus most tenants of skyscrapers are of these classes. Some skyscrapers have been built in areas where the bedrock is near surface, because this makes constructing the foundation cheaper, for example this is the case in Midtown Manhattan and Lower Manhattan, in New York City, United States, but not in-between these two parts of the city.

Today, skyscrapers are an increasingly common sight where land is expensive, as in the centers of big cities, because they provide such a high ratio of rentable floor space per unit area of land.

${\text{Simple price of floor area (currency/m²)}}={\frac {\text{price of land area (currency)}}{\text{total floor area (m²)}}}$

Environmental impact

30 St Mary Axe in London is an example of a modern environmentally friendly skyscraper.

The environmental impact of skyscrapers and whether instead of skyscrapers multiple smaller, lighter buildings would be more environmentally friendly or sustainable is under debate. The concept of a skyscraper is a product of the industrialized age, made possible by cheap fossil fuel derived energy and industrially refined raw materials such as steel and concrete. The construction of skyscrapers was enabled by steel frame construction that surpassed brick and mortar construction starting at the end of the 19th century and finally surpassing it in the 20th century together with reinforced concrete construction as the price of steel decreased and labour costs increased.

The amount of steel, concrete and glass needed to construct a single skyscraper is large, and these materials represent a great deal of embodied energy. Skyscrapers are thus energy intensive buildings, but skyscrapers have a long lifespan, for example the Empire State Building in New York City, United States completed in 1931 and is still in active use. Skyscrapers have considerable mass, which means that they must be built on a sturdier foundation than would be required for shorter, lighter buildings. Building materials must also be lifted to the top of a skyscraper during construction, requiring more energy than would be necessary at lower heights. Furthermore, a skyscraper consumes a lot of electricity because potable and non-potable water have to be pumped to the highest occupied floors, skyscrapers are usually designed to be mechanically ventilated, elevators are generally used instead of stairs, and natural lighting cannot be utilized in rooms far from the windows and the windowless spaces such as elevators, bathrooms and stairwells.

Skyscrapers can be artificially lighted and the energy requirements can be covered by renewable energy or other electricity generation of low greenhouse gas emissions. Heating and cooling of skyscrapers can be efficient, because of centralized HVAC systems, heat radiation blocking windows and small surface area of the building. There is Leadership in Energy and Environmental Design (LEED) certification for skyscrapers. For example the Empire State Building received a gold Leadership in Energy and Environmental Design rating in September 2011 and the Empire State Building is the tallest LEED certified building in the United States, proving that skyscrapers can be environmentally friendly. Also the 30 St Mary Axe in London, the United Kingdom is an environmentally friendly skyscraper.

In the lower levels of a skyscraper a larger percentage of the building cross section must be devoted to the building structure and services than is required for lower buildings:

• More structure – because it must be stronger to support more floors above
• The elevator conundrum creates the need for more lift shafts—everyone comes in at the bottom and they all have to pass through the lower part of the building to get to the upper levels.
• Building services—power and water enter the building from below and have to pass through the lower levels to get to the upper levels.

In low-rise structures, the support rooms (chillers, transformers, boilers, pumps and air handling units) can be put in basements or roof space—areas which have low rental value. There is, however, a limit to how far this plant can be located from the area it serves. The farther away it is the larger the risers for ducts and pipes from this plant to the floors they serve and the more floor area these risers take. In practice this means that in highrise buildings this plant is located on 'plant levels' at intervals up the building.

History of the tallest skyscrapers

The Empire State Building in New York City is a well-known skyscraper and was the tallest in the world for nearly 40 years.

Main article: History of the tallest buildings in the world

At the beginning of the 20th century, New York City was a center for the Beaux-Arts architectural movement, attracting the talents of such great architects as Stanford White and Carrere and Hastings. As better construction and engineering technology became available as the century progressed, New York and Chicago became the focal point of the competition for the tallest building in the world. Each city's striking skyline has been composed of numerous and varied skyscrapers, many of which are icons of 20th century architecture:

• The Flatiron Building, designed by Daniel Hudson Burnham and standing 285 ft (87 m) high, was one of the tallest buildings in the city upon its completion in 1902, made possible by its steel skeleton. It was one of the first buildings designed with a steel framework, and to achieve this height with other construction methods of that time would have been very difficult. (The 1889 Tower Building, designed by Bradford Gilbert and considered by some to be New York's first skyscraper, may have been the first building to use a skeletal steel frame.) Actually Home Insurance Building in Chicago, United States built in 1884 was the first building that had a skeletal frame. Subsequent buildings such as the Singer Building, the Metropolitan Life Tower were higher still.
• The Woolworth Building, a neo-Gothic "Cathedral of Commerce" overlooking City Hall, was designed by Cass Gilbert. At 792 feet (241 m), it became the world's tallest building upon its completion in 1913, an honor it retained until 1930, when it was overtaken by 40 Wall Street.
• That same year, the Chrysler Building took the lead as the tallest building in the world, scraping the sky at 1,046 feet (319 m). Designed by William Van Alen, an Art Deco style masterpiece with an exterior crafted of brick, the Chrysler Building continues to be a favorite of New Yorkers to this day.
• The Empire State Building, the first building to have more than 100 floors (it has 102), was completed the following year. It was designed by Shreve, Lamb and Harmon in the contemporary Art Deco style. The tower takes its name from the nickname of New York State. Upon its completion in 1931 at 1,250 feet (381 m), it took the top spot as tallest building, and towered above all other buildings until 1972. The antenna mast added in 1951 brought pinnacle height to 1,472 feet (449 m), lowered in 1984 to 1,454 feet (443 m).
• The World Trade Center officially reached full height in 1972, was completed in 1973, and consisted of two tall towers and several smaller buildings. For a short time, the first of the two towers was the world's tallest building. Upon completion, the towers stood for 28 years, until the September 11 attacks destroyed the buildings in 2001. Various governmental entities, financial firms, and law firms called the towers home.
• The Willis Tower (formerly Sears Tower) was completed in 1974, one year after the World Trade Center, and surpassed it as the world's tallest building. It was the first building to employ the "bundled tube" structural system, designed by Fazlur Khan. The building was not surpassed in height until the Petronas Towers were constructed in 1998, but remained the tallest in some categories until Burj Khalifa surpassed it in all categories in 2010. It is currently the tallest building in the United States.

Momentum in setting records passed from the United States to other nations with the opening of the Petronas Twin Towers in Kuala Lumpur, Malaysia, in 1998. The record for world's tallest building remained in Asia with the opening of Taipei 101 in Taipei, Taiwan, in 2004. A number of architectural records, including those of the world's tallest building and tallest free-standing structure, moved to the Middle East with the opening of the Burj Khalifa in Dubai, United Arab Emirates.

This geographical transition is accompanied by a change in approach to skyscraper design. For much of the twentieth century large buildings took the form of simple geometrical shapes. This reflected the "international style" or modernist philosophy shaped by Bauhaus architects early in the century. The last of these, the Willis Tower and World Trade Center towers in New York, erected in the 1970s, reflect the philosophy. Tastes shifted in the decade which followed, and new skyscrapers began to exhibit postmodernist influences. This approach to design avails itself of historical elements, often adapted and re-interpreted, in creating technologically modern structures. The Petronas Twin Towers recall Asian pagoda architecture and Islamic geometric principles. Taipei 101 likewise reflects the pagoda tradition as it incorporates ancient motifs such as the ruyi symbol. The Burj Khalifa draws inspiration from traditional Islamic art. Architects in recent years have sought to create structures that would not appear equally at home if set in any part of the world, but that reflect the culture thriving in the spot where they stand.

For current rankings of skyscrapers by height, see List of tallest buildings in the world.

The following list measures height of the roof. The more common gauge is the highest architectural detail; such ranking would have included Petronas Towers, built in 1998. See List of tallest buildings in the world for details.

Built	Building	City	Country	Roof		Floors	Pinnacle		Current status
1870	Equitable Life Building	New York City	United States	142 ft	43 m	8			Destroyed by fire in 1912
1889	Auditorium Building	Chicago	United States	269 ft	82 m	17	349 ft	106 m	Standing
1890	New York World Building	New York City	United States	309 ft	94 m	20	349 ft	106 m	Demolished in 1955
1894	Manhattan Life Insurance Building	New York City	United States	348 ft	106 m	18			Demolished in 1963
1895	Milwaukee City Hall	Milwaukee	United States	353 ft	108 m	15			Standing
1899	Park Row Building	New York City	United States	391 ft	119 m	30			Standing
1901	Philadelphia City Hall	Philadelphia	United States	511 ft	155.8 m	9	548 ft	167 m	Standing
1908	Singer Building	New York City	United States	612 ft	187 m	47			Demolished in 1968
1909	Met Life Tower	New York City	United States	700 ft	213 m	50			Standing
1913	Woolworth Building	New York City	United States	792 ft	241 m	57			Standing
1930	40 Wall Street	New York City	United States			70	927 ft	283 m	Standing
1930	Chrysler Building	New York City	United States	927 ft	282.9 m	77	1,046 ft	319 m	Standing
1931	Empire State Building	New York City	United States	1,250 ft	381 m	102	1,454 ft	443 m	Standing
1972	World Trade Center (North tower)	New York City	United States	1,368 ft	417 m	110	1,727 ft	526.3 m	Destroyed in 2001
1974	Willis Tower (formerly Sears Tower)	Chicago	United States	1,450 ft	442 m	108	1,729 ft	527 m	Standing
2004	Taipei 101	Taipei	Taiwan	1,474 ft	449 m	101	1,671 ft	509 m	Standing
2008	Shanghai World Financial Center	Shanghai	China	1,599 ft	487 m	101	1,614 ft	492 m	Standing
2010	Burj Khalifa	Dubai	United Arab Emirates	2,717 ft	828 m	160	2,717 ft	828 m	Standing

Source: emporis.com

• 
  Taipei 101, formerly the world's tallest skyscraper, was the first to exceed the half-kilometer mark.
• 
  The iconic World Trade Center twin towers were destroyed in 2001.
• 
  The Willis Tower in Chicago was the world's tallest building from 1974 to 1998, and remains the tallest in the Western Hemisphere.
• 
  The Petronas Twin Towers.
• 
  Tower 2 of the International Finance Centre in Hong Kong is one of the 20 tallest buildings in the world.

Future developments

At the time Taipei 101 broke the half-km mark in height, it was already technically possible to build structures towering over a km above the ground. Proposals for such structures have been put forward, including the Kingdom Tower to be built in Jeddah, Saudi Arabia and Burj Mubarak Al Kabir in Kuwait. Kilometer-plus structures present architectural challenges that may eventually place them in a new architectural category.

The following skyscrapers, all contenders for being among the tallest in their city or region, are under construction and due to be completed in the next few years:

• Construction of the 133-floor, 640 m tall Digital Media City Landmark Building in Digital Media City, Seoul, South Korea, started in 2009, which will be the second-tallest building in the world when it is completed in 2015, housing the world's tallest observatory and hotels. Being constructed at the fastest speed among major skyscraper projects by South Korea's Samsung C&T (who also built Burj Khalifa), the supertall is the first skyscraper to contain an entire city inside a building, including the world's largest aquarium, a luxury department store, shopping malls, clinic center, high-tech offices, first-class apartments, six to eight-star hotels, a concert restaurant, a broadcasting studio and an art center.
• Construction of the Shanghai Tower started on 29 November 2008. The tower will be 632 m (2,073 ft) high and have 127 floors. The building will feature a glass curtain wall and nine indoor gardens when it is completed in 2014.
• Construction of the 151-floor, 610 m tall 151 Incheon Tower in Songdo International City, Incheon, South Korea, started in 2008, which will be the tallest twin towers in the world when it is completed in 2014.
• The Abraj Al-Bait Towers, also known as the "Mecca Royal Clock Hotel Tower" is a complex under construction in Mecca, Saudi Arabia by the Saudi Binladin Group. The complex consists of seven towers, and the tallest tower (Hotel Tower) will have a height of 601 m (1,972 ft). Upon completion in 2011, the structure will have the largest floor area of any structure in the world, at 1,500,000 m² (16,000,000 sq ft).
• Construction of the 110-floor, 510 m tall Busan Lotte World, Busan, South Korea, started in 2009. It is due for completion in 2016.
• One World Trade Center is currently under construction in New York City and will be the tallest tower in the redevelopment of the site of the former World Trade Center. Its pinnacle will reach a height of 541.4 m (1,776 ft), a height (in feet) representing the year of the United States Declaration of Independence.
• World One is a 442 m (1,450 ft) tall residential skyscraper under construction in Mumbai, India. It is located in Upper Worli of Mumbai on a 17.5 acre site. The project will cost INR 2,000 crore (US$380 million), be completed by 2014 and will have the world’s second tallest residential tower once completed. It will be rated as Leed Gold Certified building by the Green Building Council. World One is designed by Pei Cobb Freed and Partners and Leslie E. Robertson Associates.
• Construction of The Shard in London, United Kingdom started in March 2009, and was completed in May 2012, in time for the London Olympics. At 310 m (1,017 ft), it is set to be the tallest building in the European Union.
• Gran Torre Santiago will be the tallest building in Latin America with 300 m (984 ft) to the top. Construction started in 2006 and is planned to finish in 2013. It is located in Chile´s Capital Santiago and is being developed by Cencosud.

See also

• Skyscraper design and construction
• Skyscraper Index
• Emporis Skyscraper Award
• Skyscraper Museum in NYC
• Skyline
• List of tallest buildings and structures in the world
• List of tallest buildings in the world
• Timeline of three tallest structures in the world
• List of cities with most skyscrapers
• Skyscrapers in film
• Groundscraper
• Vertical farming, "farmscrapers"
• Seascraper
• World's littlest skyscraper

Further reading

• Skyscrapers: Form and Function, by David Bennett, Simon & Schuster, 1995.
• Landau, Sarah Bradford; Condit, Carl W., Rise of the New York skyscraper, 1865–1913, New Haven : Yale University Press, 1996. ISBN 0-300-06444-6
• Willis, Carol, Form Follows Finance: Skyscrapers and Skylines in New York and Chicago. Princeton Architectural Press, 1995. 224 P. ISBN 1-56898-044-2

External links

• Historical photos of skyscrapers in New York City
• Skyscraper Museum
• Tallest Building in the World
• SkyscraperPage Technical information and diagrams
• AllAboutSkyscrapers.com Articles, Data and Photos
• Skyscrapercity Technical information and Project Update forum
• Template:Dmoz
• 1880s "skyscraper" citations from word researcher Barry Popik.