Editing Skyscraper (section)

==Design and construction==
{{Main|Skyscraper design and construction}}
[[File:The_Sky_Garden.jpg|thumb|The Sky Garden in London's [[20 Fenchurch Street]] is an example of where top floors are used as restaurants or viewing areas, and in this case, both.]]
[[File:Shanghai_skyscrapers_5166285.jpg|thumb|Contemporary skyscrapers in [[Shanghai]]]]
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]], [[Civil Engineering|engineering]], and [[construction]] management.

One common feature of skyscrapers is 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 them to be built taller than typical load-bearing walls of reinforced concrete. Skyscrapers usually have a particularly small surface area of what are conventionally thought of as walls. Because the walls are not load-bearing most skyscrapers are characterized by surface areas of windows made possible by the concept of steel frame and curtain wall. However, skyscrapers can also have curtain walls that mimic conventional walls and have a small surface area of windows.

The concept of a skyscraper is a product of the [[Industrial society|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 labor costs increased.

The steel frames become inefficient and uneconomic for supertall buildings as usable floor space is reduced for progressively larger supporting columns.<ref name="lehigh.edu">{{cite web|url=http://www.lehigh.edu/~infrk/2011.08.article.html |first1=Yasmin S. |last1=Khan |title= Fazlur Rahman Khan Distinguished Lecture Series |publisher=Lehigh University |access-date=14 June 2013}}</ref> Since about 1960, tubular designs have been used for high rises. This reduces the usage of material (more efficient in economic terms – [[Willis Tower]] uses a third less steel than the Empire State Building) yet allows greater height. It allows fewer interior columns, and so creates more usable floor space. It further enables buildings to take on various shapes.

[[Elevator]]s 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 Elevator Company|Otis]], [[ThyssenKrupp AG|ThyssenKrupp]], [[Schindler Group|Schindler]], and [[KONE]].

Advances in construction techniques have allowed skyscrapers to narrow in width, while increasing in height. Some of these new techniques include [[Tuned mass damper|mass dampers]] to reduce vibrations and swaying, and gaps to allow air to pass through, reducing wind shear.<ref>{{cite news |title=Why Can't We Build Skinny Skyscrapers Everywhere?|url=http://www.citylab.com/design/2014/06/why-cant-we-build-skinny-skyscrapers-everywhere/373493/ |first1= Kriston |last1=Capps |newspaper=Bloomberg.com|date=26 June 2014|access-date=31 December 2015}}</ref>

===Basic design considerations===
[[File:Jade Signature under construction September 2016.jpg|thumb|A view of the interior structural design can be seen in this residential skyscraper that was constructed in [[Florida]], the [[List of tallest buildings in Sunny Isles Beach|Jade Signature]].]]
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 tremendous damage such failure would cause. This presents a paradox to [[Civil engineering|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; instead, one 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===
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 [[Setback (architecture)|setbacks]] are actually a result of the building code at the time ([[1916 Zoning Resolution]]), 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 supertall 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.

===Steel frame===
By 1895, [[steel]] had replaced [[cast iron]] as skyscrapers' structural material. Its malleability allowed it to be formed into a variety of shapes, and it could be riveted, ensuring strong connections.<ref>{{cite journal |title=Built Like Bridges: Iron, Steel, and Rivets in the Nineteenth-century Skyscraper|author1-link=Thomas Leslie (architect) |first=Thomas |last=Leslie |journal=Journal of the Society of Architectural Historians|volume=69 |issue=2 |date=June 2010|pages=234–261 |jstor=10.1525/jsah.2010.69.2.234|doi=10.1525/jsah.2010.69.2.234 }} Abstract only.</ref> The simplicity of a steel frame eliminated the inefficient part of a shear wall, the central portion, and consolidated support members in a much stronger fashion by allowing both horizontal and vertical supports throughout. Among steel's drawbacks is that as more material must be supported as height increases, the distance between supporting members must decrease, which 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.<ref name="lehigh.edu"/>

===Tube structural systems===
{{See also|Tube (structure)}}
[[File:Willis_Tower_From_Lake.jpg|thumb|The [[Willis Tower]] in Chicago visibly expresses the bundled tube frame. Tube frame variations are commonly used in tall modern skyscapers.|400x400px]]
A new structural system of framed tubes was developed by [[Fazlur Rahman Khan]] in 1963. The framed tube structure is defined 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".<ref>{{cite journal |last1=Ali |first1=Mir M. |title=Evolution of Concrete Skyscrapers |journal=Electronic Journal of Structural Engineering |date=January 2001 |volume=1 |issue=1 |pages=2–14 |doi=10.56748/ejse.1111 |s2cid=251690475 |doi-access=free }}</ref><ref>{{cite journal |last1=Khan |first1=Fazlur Rahman |author1-link=Fazlur Rahman Khan |last2=Rankine |first2=J. |title=Structural Systems |journal=Tall Building Systems and Concepts |publisher=[[Council on Tall Buildings and Urban Habitat]], [[American Society of Civil Engineers]] |date=1980 |volume=SC |page=42}}</ref> Closely spaced interconnected exterior columns form the tube. Horizontal loads (primarily wind) are supported by the structure as a whole. Framed tubes allow fewer interior columns, and so create more usable floor space, and about half the exterior surface is available for windows. 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 allowing buildings to reach greater heights. Concrete tube-frame construction<ref name=Ali/> was first used in the [[DeWitt-Chestnut Apartment Building]], completed in [[Chicago]] in 1963,<ref>{{cite encyclopedia|author=Alfred Swenson & Pao-Chi Chang|title=building construction|encyclopedia=[[Encyclopædia Britannica]] |year=2008|url=http://www.britannica.com/EBchecked/topic/83859/building-construction|access-date=9 December 2008}}</ref> and soon after in the [[John Hancock Center]] and [[construction of the World Trade Center|World Trade Center]].

The [[tube (structure)|tubular systems]] are fundamental to tall building design. Most buildings over 40 stories constructed since the 1960s now use a tube design derived from Khan's structural engineering principles,<ref name="lehigh.edu"/><ref name="constructionweekonline.com">{{cite news|url=http://www.constructionweekonline.com/article-9180-top-10-worlds-tallest-steel-buildings/1/print/ |title=Top 10 world's tallest steel buildings |newspaper=Construction Week Online |date=27 September 2010 |publisher=Constructionweekonline.com |access-date=14 June 2013}}</ref> examples including the [[construction of the World Trade Center]], [[Aon Center (Chicago)|Aon Center]], [[Petronas Towers]], [[Jin Mao Building]], and most other supertall skyscrapers since the 1960s.<ref name=Ali/> The strong influence of tube structure design is also evident in the construction of the current tallest skyscraper, the [[Burj Khalifa]],<ref name=Bayley>{{cite news|title=Burj Dubai: The new pinnacle of vanity|author=Stephen Bayley|work=[[The Daily Telegraph]]|date=5 January 2010|url=https://www.telegraph.co.uk/news/worldnews/middleeast/dubai/6934603/Burj-Dubai-The-new-pinnacle-of-vanity.html |archive-url=https://ghostarchive.org/archive/20220111/https://www.telegraph.co.uk/news/worldnews/middleeast/dubai/6934603/Burj-Dubai-The-new-pinnacle-of-vanity.html |archive-date=11 January 2022 |url-access=subscription |url-status=live|access-date=26 February 2010}}{{cbignore}}</ref> which uses a [[Buttressed core]].<ref name="academic.csuohio.edu">{{cite web|last1=Baker|first1=William|last2=Pawlikowski|first2=James|title=Higher and Higher: The Evolution of the Buttressed Core|url=http://academic.csuohio.edu/duffy_s/CVE_601_Struct_1.pdf|website=academic.csuohio.edu|access-date=4 April 2017|archive-date=10 August 2017|archive-url=https://web.archive.org/web/20170810080936/http://academic.csuohio.edu/duffy_s/CVE_601_Struct_1.pdf|url-status=dead}}</ref>

'''Trussed tube and X-bracing:'''
[[File:Skyscraper structure.png|thumb|left|Changes of structure with height; the [[Tube (structure)|tubular systems]] are fundamental for supertall buildings.]]
Khan pioneered several other variations of the tube structure design. One of these was the concept of [[X-bracing]], or the [[Tube (structure)#Trussed tube|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 Expressionism|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 [[seismic loading|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.

The [[John Hancock Center]] was far more efficient than earlier [[Steel frame|steel-frame]] structures. Where the [[Empire State Building]] (1931), required about 206 kilograms of steel per square metre and [[28 Liberty Street]] (1961) required 275, the John Hancock Center required only 145.<ref name=Britannica/> The trussed tube concept was applied to many later skyscrapers, including the [[Onterie Center]], [[Citigroup Center]] and [[Bank of China Tower, Hong Kong|Bank of China Tower]].<ref name="Introduction to Tall Building architectures">{{cite web|url=http://teaching.ust.hk/~civl101/Civl101%20-%20Introduction%20to%20Tall%20Building%20Structures.pdf |page=34 |title=Introduction to Tall building Structures |author=D. M Chan |publisher=Teaching.ust.hk |url-status=dead |archive-url=https://web.archive.org/web/20101217063145/http://teaching.ust.hk/~civl101/Civl101%20-%20Introduction%20to%20Tall%20Building%20Structures.pdf |archive-date=17 December 2010 }}</ref>
[[File:HK Bank of China Tower 2008 (2).jpg|thumb|356x356px|The [[Bank of China Tower (Hong Kong)|Bank of China Tower]] in Hong Kong uses a trussed tube design]]

'''Bundled tube:'''
An important variation on the tube frame is the [[Tube (structure)#Bundled tube|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 bundled tube structure meant that "buildings no longer need be boxlike in appearance: they could become sculpture."<ref name=Bayley/>

'''Tube in tube:'''
Tube-in-tube system takes advantage of core shear wall tubes in addition to exterior tubes. The inner tube and outer tube work together to resist gravity loads and lateral loads and to provide additional rigidity to the structure to prevent significant deflections at the top. This design was first used in [[One Shell Plaza]].<ref>{{cite web |url=http://khan.princeton.edu/khanOneShell.html |title=One Shell Plaza - Fazlur Khan - Structural Artist of Urban Building Forms |publisher=Khan.princeton.edu |access-date=18 June 2014 |archive-date=1 October 2022 |archive-url=https://web.archive.org/web/20221001140950/https://khan.princeton.edu/khanOneShell.html |url-status=dead }}</ref> Later buildings to use this structural system include the [[Petronas Towers]].<ref>{{cite book |url=https://books.google.com/books?id=K792dXxSI4UC&q=tube+in+tube+petronas+tower&pg=PA24 |title=Structures in the New Millennium - Google Books |date=January 1997 |access-date=18 June 2014 |isbn=9789054108986 |last1=Lee |first1=P. K. K.|publisher=CRC Press }}</ref>

'''Outrigger and belt truss:'''
The outrigger and belt truss system is a lateral load resisting system in which the tube structure is connected to the central core wall with very stiff outriggers and belt trusses at one or more levels.<ref name="support1">{{cite web |url=http://www.support.tue.nl/archief/studiereizen/studiereis2007/pudong_swf_en.htm |title=SUPport Studytour 2007 |publisher=Support.tue.nl |access-date=18 June 2014 |url-status=dead |archive-url=https://web.archive.org/web/20140714182328/http://www.support.tue.nl/archief/studiereizen/studiereis2007/pudong_swf_en.htm |archive-date=14 July 2014 }}</ref> [[140 William Street, Melbourne|BHP House]] was the first building to use this structural system followed by the First Wisconsin Center, since renamed [[U.S. Bank Center (Milwaukee)|U.S. Bank Center]], in Milwaukee. The center rises 601 feet, with three belt trusses at the bottom, middle and top of the building. The exposed belt trusses serve aesthetic and structural purposes.<ref name="princeton1">{{cite web |url=http://khan.princeton.edu/works.html |title=Major Works - Fazlur Khan - Structural Artist of Urban Building Forms |publisher=Khan.princeton.edu |date= |access-date=18 June 2014 |archive-date=22 May 2015 |archive-url=https://web.archive.org/web/20150522035020/http://khan.princeton.edu/works.html |url-status=dead }}</ref> Later buildings to use this include [[Shanghai World Financial Center]].<ref name="support1"/>

'''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.<ref name=Ali/> [[Trump Tower]] in New York City is also another example that adapted this system.<ref>{{cite journal |last1=Seinuk |first1=Ysrael A. |last2=Cantor |first2=Irwin G. |title=Trump Tower: Concrete Satisfies Architectural, design, and construction demands |journal=Concrete International |date=March 1984 |volume=6 |issue=3 |pages=59–62 |url=http://www.concrete.org/Publications/InternationalConcreteAbstractsPortal.aspx?m=details&i=9220 |language=en |issn=0162-4075}}</ref>

'''Shear wall frame interaction system:'''
[[File:Cook County Administration Building (9181641122) (2).jpg|thumb|The [[Cook County Administration Building]] in Chicago was the first to utilize a shear wall frame interaction system|200px]]
Khan developed the shear wall frame interaction system for mid high-rise buildings. This structural system uses combinations of shear walls and frames designed to resist lateral forces.<ref>{{cite web |url=http://www2.iccsafe.org/states/newyorkcity/Building/PDFs/Chapter%2016_Structural%20Design.pdf |title=0a_copy_NYC_2008_IBC.vp |access-date=18 June 2014 |archive-date=28 August 2017 |archive-url=https://web.archive.org/web/20170828183101/http://www2.iccsafe.org/states/newyorkcity/building/pdfs/chapter%2016_structural%20design.pdf |url-status=dead }}</ref> The first building to use this structural system was the 35-stories Brunswick Building.<ref name="princeton1"/> The Brunswick building (today known as the "[[Cook County Administration Building]]") was completed in 1965 and became the tallest reinforced concrete structure of its time. The structural system of Brunswick Building consists of a concrete shear wall core surrounded by an outer concrete frame of columns and spandrels.<ref>{{cite web |url=http://khan.princeton.edu/khanBrunswick.html |title=Brunswick Building - Fazlur Khan - Structural Artist of Urban Building Forms |publisher=Khan.princeton.edu |access-date=18 June 2014 |archive-date=1 October 2022 |archive-url=https://web.archive.org/web/20221001134230/https://khan.princeton.edu/khanBrunswick.html |url-status=dead }}</ref> Apartment buildings up to 70 stories high have successfully used this concept.<ref>{{cite web |author=Civil Engineer |url=http://www.civilengineergroup.com/shear-wallframe-interaction.html |title=Shear Wall-Frame Interaction |publisher=Civil Engineering Group |date=12 March 2011 |access-date=18 June 2014 |url-status=dead |archive-url=https://archive.today/20140618142806/http://www.civilengineergroup.com/shear-wallframe-interaction.html |archive-date=18 June 2014 }}</ref>

===The elevator conundrum===
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.<ref name="HSW3">{{cite web|url=http://science.howstuffworks.com/skyscraper3.htm|title=How Skyscrapers Work: Making it Functional|date=3 April 2001|publisher=HowStuffWorks|access-date=30 October 2008}}</ref>
[[File:HK Wan Chai North 中環廣場 Central Plaza 26th floor sky lift lobby October 2018 SSG 01.jpg|thumb|Sky lobby at [[Central Plaza (Hong Kong)|Central Plaza]] in [[Hong Kong]] which has clear signage of the floors served by the different elevators.]]
Many tall buildings use elevators in a non-standard configuration to reduce their footprint. Buildings such as the former [[World Trade Center (1973–2001)|World Trade Center Towers]] and Chicago's [[John Hancock Center]] use [[sky lobby|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 elevator]]s, 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 person on one level needs to get off at a given floor.

Buildings with sky lobbies include the [[World Trade Center (2001–present)|World Trade Center]], [[Petronas Twin Towers]], [[Willis Tower]] and [[Taipei 101]]. The 44th-floor sky lobby of the John Hancock Center also featured the first [[high-rise]] indoor [[swimming pool]], which remains the highest in the United States.<ref name=Emporis>{{cite web|url=http://www.emporis.com/en/wm/bu/?id=116876|archive-url=https://web.archive.org/web/20040415075239/http://www.emporis.com/en/wm/bu/?id=116876|url-status=usurped|archive-date=15 April 2004|title=John Hancock Center|author=Emporis GmbH}}</ref>