Editing Skyscraper (section)

===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>