Editing Willis Tower (section)

== Architecture ==
{{Stack|[[File:Willis Tower tube structure.svg|thumb|Breakdown of the [[Tube (structure)#Bundled tube|bundled tube structure]] of Willis Tower with simplified floor plans]]}}
The Willis Tower was designed by architect [[Bruce Graham]] and structural engineer [[Fazlur Rahman Khan]] of Skidmore, Owings and Merrill.<ref name="PL p. 181">{{harvnb|Pridmore|Larson|2018|ps=.|p=181}}</ref> Graham and Khan designed the building as nine square "[[Tube (structure)|tubes]]", clustered in a 3×3 matrix forming a square base with {{convert|225|ft|m|adj=on}} sides.<ref name="n108217158" /><ref name=":2" />  The building's rentable area is {{convert|3810000|ft2|m2|-2|abbr=on}}.<ref>{{Cite web |title=Willis Tower |url=https://www.som.com/projects/willis-tower-formerly-sears-tower/ |access-date=2024-07-22 |website=SOM |language=en-US}}</ref> The structure was intended to accommodate 16,500 employees.<ref name="p511514170">{{cite news |last=Hoyt |first=Monty |date=June 22, 1973 |title=Sears Tower, world's tallest, may also be world's safest: No alternative to safety "Bundled tube" design |page=12 |work=The Christian Science Monitor |id={{ProQuest|511514170}}}}</ref>

=== Form and facade ===
Each of the "tubes" is a column-free module measuring {{convert|75|by|75|ft}},<ref>{{cite web |date=September 2, 2007 |title=Sears Tower |url=http://www.tallestbuildingintheworld.com/building_id_5_Sears+Tower.php |url-status=dead |archive-url=https://web.archive.org/web/20090226193756/http://www.tallestbuildingintheworld.com/building_id_5_Sears+Tower.php |archive-date=February 26, 2009 |access-date=September 14, 2009 |work=TallestBuildingintheWorld.com}}</ref> which [[Setback (architecture)|set back]] at different stories.<ref name="PL p. 182" /> There are setbacks at the 50th, 66th, and 90th floors.<ref name="Drexler Menges 2009 p." /><ref name="Žaknić Smith Rice Council on Tall Buildings and Urban Habitat p.">{{cite book |last1=Žaknić |first1=Ivan |title=100 of the world's tallest buildings |last2=Smith |first2=Matthew |last3=Rice |first3=Dolores B. |author4=Council on Tall Buildings and Urban Habitat |date=1998 |publisher=Gingko Press |isbn=3-927258-60-1 |page=207 |oclc=40110184}}</ref><ref name="p148369539" /> The lowest 50 stories contain nine tubes and cover {{convert|52000|ft2}} each.<ref name="n108217158" /><ref name=":2" /><ref name="n109122999" /> The northwest and southeast tubes terminate at the 50th floor. The 51st through 66th floors each span {{convert|41420|ft2}}, above which the northeast and southwest tubes end. From the 67th to 90th floors, each story is shaped like a cross, covering {{convert|30170|ft2}}. The north, east, and south tubes end at the 90th floor; the remaining west and center tubes reach 108 floors, with an area of {{convert|12283|ft2}} on each of the top stories.<ref name="n108217158" />
[[File:FR khan sculputure at Sears tower.jpg|thumb|right|Sculpture honoring [[Fazlur Rahman Khan]], considered the father of tubular designs, at the Willis Tower. Khan is known for making important advancements in skyscraper engineering.]]
The Sears Tower was the first building to use this innovative design. It was both structurally efficient and economic: at 1,450 feet, it provided more space and rose higher than the [[Empire State Building]] and cost much less per unit area.<ref name="Lynn S.Beadle pays tribute to Khan">{{cite book |author=National Academy of Engineering |url=https://books.google.com/books?id=vD0rAAAAYAAJ |title=Memorial Tributes |publisher=National Academies |year=1979 |isbn=9780309034821 |id=NAP:14723}}</ref> The system would prove highly influential in skyscraper construction and has been used in most [[supertall]] buildings since, including the world's current tallest building, the [[Burj Khalifa]].<ref>{{cite news |title=Burj Dubai: The new pinnacle of vanity |newspaper=Telegraph |url=https://www.telegraph.co.uk/news/worldnews/middleeast/dubai/6934603/Burj-Dubai-The-new-pinnacle-of-vanity.html |url-status=live |url-access=subscription |access-date=February 25, 2014 |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=January 11, 2022}}{{cbignore}}</ref><ref>{{cite news |title=Top 10 world's tallest steel buildings |newspaper=Construction Week Online |date=September 27, 2010 |url=http://www.constructionweekonline.com/article-9180-top-10-worlds-tallest-steel-buildings/1/print/ |access-date=February 25, 2014 |publisher=Constructionweekonline.com}}</ref> In February 1982, two television antennas were added to the structure, increasing its total height to {{convert|1707|ft|m|1}}. The western antenna was later extended, bringing the overall height to {{convert|1729|ft|m|1}}<ref name="skyscraperCenter" /> on June 5, 2000, to improve reception of local [[NBC]] station [[WMAQ-TV]].

The perimeter of the Willis Tower contains columns that are spaced {{convert|15|ft}} apart on their centers. The [[facade]] is made of anodized aluminum and black glass.<ref name="Drexler Menges 2009 p." /><ref name="Žaknić Smith Rice Council on Tall Buildings and Urban Habitat p." /> [[Alcoa]] manufactured {{convert|4|e6lb}} of aluminum sheeting for the building's facade.<ref>{{Cite news |date=August 9, 1971 |title=Alcoa to Supply Sears Tower Aluminum Sheet |pages=68 |work=Chicago Tribune |url=https://www.newspapers.com/clip/109143707/alcoa-to-supply-sears-tower-aluminum/ |access-date=September 7, 2022}}</ref> Black bands appear on the tower around the 29th–32nd, 64th–65th, 88th–89th, and 104th–108th floors. These elements are louvers to ventilate the building's environmental support systems and obscure its [[truss|belted trusses]].<ref name=":3">''The Times-News'', February 15, 1975</ref> The rest of the facade is made of 16,000 rectangular windows.<ref name="p170442312">{{Cite news |last=Nagelberg |first=Alvin |date=June 24, 1973 |title=Going up:: a fantastic elevator voyage The Sears Tower Almanac 14 Sears Escalators link the 33d and 34th floors |page=I14 |work=Chicago Tribune |id={{proQuest|170442312}}}}</ref><ref name="p148412303">{{cite news |date=June 2, 1973 |title=Sears Tower Now Tallest In Nation |page=E12 |work=The Washington Post, Times Herald |issn=0190-8286 |id={{ProQuest|148412303}}}}</ref> all of which measure {{convert|5|by|8|ft}} and are tinted with bronze.<ref name="p148412303" />

Outside the building, there was originally a {{convert|80000|ft2|adj=on}} plaza made of pink granite.<ref name="p170442312" /> In the late 2010s, a three-level wing was built along the western and southern sides of the tower replacing the plaza. The [[roof garden]] above the annex spans {{convert|30000|ft2}}.<ref name="architecturalrecord.com" /><ref name="Chicago YIMBY 2020">{{cite web |date=October 22, 2020 |title=Final Touches Remain for Catalog, Willis Tower's New Podium in The Loop |url=https://chicagoyimby.com/2020/10/final-touches-remain-for-catalog-willis-towers-new-podium-in-the-loop.html |access-date=September 7, 2022 |website=Chicago YIMBY}}</ref> The annex contains a facade of black steel and aluminum, similar to in the original building.<ref name="architecturalrecord.com" /> The Jackson Boulevard facade of the annex contains an artwork by Olafur Eliasson, entitled ''Atmospheric wave wall''.<ref name="Willis Tower Art of the Neighborhood New">{{cite web |title=Art of the Neighborhood New |url=https://www.willistower.com/artoftheneighborhood |access-date=September 7, 2022 |website=Willis Tower}}</ref><ref name="Nast 2021">{{cite web |last=Li |first=Jennifer S. |date=January 13, 2021 |title=Olafur Eliasson's New Chicago Art Installation Captures Shifting Sunlight and a Changing Climate |url=https://www.architecturaldigest.com/story/olafur-eliassons-chicago-new-art-installation-captures-shifting-sunlight-and-a-changing-climate |access-date=September 7, 2022 |website=Architectural Digest}}</ref> The work, measuring {{convert|30|by|60|ft}} across,<ref name="Willis Tower Art of the Neighborhood New" /> comprises almost 2,000 blue-and-green steel tiles, which are decorated with hexagonal motifs.<ref name="Nast 2021" /><ref name="juliana neira I designboom 2021">{{cite web |last=Neira |first=Juliana |date=January 20, 2021 |title=Olafur Eliasson Installs 1,963 Dynamic Metal Tiles at Chicago's Willis Tower |url=https://www.designboom.com/art/olafur-eliasson-atmospheric-wave-wall-willis-tower-chicago-01-21-2021/ |access-date=September 7, 2022 |website=designboom}}</ref> The wall is backlit at night.<ref name="Willis Tower Art of the Neighborhood New" /><ref name="Nast 2021" />

=== Structural and mechanical features ===
The interior includes {{convert|74000|ST|LT t}} of steel, {{convert|4|e6lb}} of aluminum, and {{convert|101|acre|ft2 m2}} of concrete flooring.<ref name="p148412303" /><ref name="p133809828" /> The building contains diagonal columns only on the two stories immediately below each of the setbacks, thus reducing shear stress.<ref name="Žaknić Smith Rice Council on Tall Buildings and Urban Habitat p." /> The interior of the building could not contain diagonal beams, since these would have obstructed the connections between each of the "tubes". Therefore, the columns and the horizontal beams on each story are connected by rigid joints.<ref name="Drexler Menges 2009 p." /> The superstructure was designed to withstand wind gusts of {{convert|130|mph}}, which on average would occur once every hundred years.<ref name="p511514170" /> According to the ''Chicago Tribune'', the top of the building would be able to bend by as much as {{convert|7|in}}, returning to its normal position within 7.2 seconds.<ref name="p170442312" />

The Willis Tower's basement extends {{Convert|50|ft}} deep, resting on a {{convert|5|ft|m|-thick|adj=mid}} concrete slab. The ground directly beneath the building was largely made of clay; the underlying layer of limestone was as much as {{Convert|100|ft}} beneath ground level.<ref name="p169075911" /> As a result, the foundation was excavated using 201 [[Caisson (engineering)|caissons]],<ref name="p169075911" /><ref name="p1521729590" /> of which 114 reached the underlying limestone.<ref name="p1521729590" /> The caissons created holes that measured up to {{convert|10|ft}} across. Some holes at the northwestern and northeastern corners of the site filled up with groundwater and had to be drained. Workers next placed steel tubes into the holes, then poured concrete around the tubes.<ref name="p169075911" />

During the Sears Tower's construction, SOM and Chicago government officials considered adding "smoke free and fire free" areas to the building, as well as a complete sprinkler system serving all floors. Neither of these features had previously been used in a structure in Chicago.<ref name="n109143307">{{Cite news |last=Zahour |first=Frank |date=January 3, 1971 |title=City Planners Consider Pioneer Ideas for Including Fire Safety in Sears Tower |pages=34 |work=Chicago Tribune |url=https://www.newspapers.com/clip/109143307/city-planners-consider-pioneer-ideas/ |access-date=September 7, 2022}}</ref> Even though regulations did not require a fire sprinkler system, the building was equipped with one from the beginning. There are around 40,000 sprinkler heads in the building, installed at a cost of $4 million.<ref name=":3" /> When it was completed, the Sears Tower was heated electrically, unlike older structures that used gas heating.<ref name="n109146209">{{Cite news |last=Nagelberg |first=Alvin |date=January 27, 1971 |title=Sears Tower Goes Electric; Standard Turns to Gas |pages=53 |work=Chicago Tribune |url=https://www.newspapers.com/clip/109146209/sears-tower-goes-electric-standard/ |access-date=September 7, 2022}}</ref> It included 145,000 light fixtures and a cooling system capable of 17,000 tons of refrigeration.<ref name="p133809828" /><ref name="p148369539" /> Furthermore, the tower contained fire-suppression and communications systems for emergency use, which were powered by diesel generators.<ref name="p511514170" /><ref name="n109180857">{{Cite news |date=March 11, 1973 |title=Sears Tower betters code |pages=551 |work=Chicago Tribune |url=https://www.newspapers.com/clip/109180857/sears-tower-betters-code/ |access-date=September 8, 2022}}</ref> If there was a fire in one section of the building, the building's smoke-detection system would close off the fresh-air intake openings in that section, discharging smoke outdoors.<ref name="p511514170" />

Fifteen above-ground stories, as well as three of the basement levels, contain mechanical equipment. Above the Skydeck on the 103rd floor is a seven-story mechanical penthouse.<ref name="p148369539" />

==== Elevators and escalators ====
The Sears Tower was planned with 103 elevators,<ref name="p511514170" /><ref name="p170442312" /> including 14 [[double-deck elevator]]s.<ref name="p511514170" /><ref name="p169950437">{{Cite news |date=October 11, 1970 |title=Sears Tower: What the Pedestrian Will See |page=C1 |work=Chicago Tribune |id={{proQuest|169950437}}}}</ref> The office stories are served by 97 elevator cabs; due to the presence of the double-deck elevators, these occupy 83 shafts.<ref name="Ori Brinson 2018" /> As designed, one bank of single-deck elevators connected the lobby to the lowest 28 stories.<ref name="p169950437" /> Banks of double-deck elevators traveled to "sky lobbies" at the 33rd/34th and 66th/67th floors, where passengers could transfer to local elevators. The 34th through 103rd stories were served by local elevators that operated from the sky lobbies.<ref name="p511514170" /><ref name="p169950437" /> Two elevators also ran directly from the lobby to the Skydeck on the 103rd floor.<ref name="n109146093">{{Cite news |last=Nagelberg |first=Alvin |date=April 11, 1971 |title=Glass Elevators Give Hotel Lift |pages=49 |work=Chicago Tribune |url=https://www.newspapers.com/clip/109146093/glass-elevators-give-hotel-liftalvin/ |access-date=September 7, 2022}}</ref> {{As of|2018}}, the elevators carried 5.8 million passengers per year.<ref name="Koziarz 20182" /><ref name="Ori Brinson 2018" />

Six of the elevators are used for freight.<ref name="p170442312" /> One of the freight elevators served all stories, traveling to a height of {{convert|1440|ft}}.<ref name="n109146093" /> During a fire or another emergency, this elevator would be reserved for the [[Chicago Fire Department]]. Other elevators would be controlled from the 33rd floor. During a fire, elevators would be dispatched to the affected floors to assist with evacuation.<ref name="p511514170" />

The building also had 16 escalators, including a set of double-height escalators that traveled from the main lobby to the lower mezzanine.<ref name="p169950437" /> Another set of escalators connects the 33rd and 34th stories.<ref name="p170442312" />

=== Interior ===

==== Base ====
[[File:Base of the Willis Tower (2021).jpg|thumb|223x223px|Bottom of the tower]]
When the building was completed, the main entrance was on Wacker Drive to the west. There was a plaza on the south side of the building, sloping upward toward Franklin Street to the east. The Franklin Street side of the building was {{Convert|6|ft}} lower than the Wacker Drive entrance, so the entrances on Franklin Street were actually below the plaza, leading to the building's lower mezzanine.<ref name="p169950437" /> Below ground level are three basement levels with a total area of {{convert|400000|ft2}}.<ref name="nyt-1970-10-18" /> The basements included a 1,200-seat cafeteria, commercial space, service areas, and a loading dock for 17 trucks.<ref name="Drexler Menges 2009 p." /> The basement also contained a 150-spot parking garage.<ref name="p170442312" />

{{As of|2022}}, the building's base covers {{convert|463000|ft2}} and contains two lobbies for tenants.<ref name="Rogal 2022" /><ref name="architecturalrecord.com" /> The building's tenants primarily enter from Wacker Drive and Franklin Street. Shoppers, restaurant patrons, and visitors to the Skydeck observation deck use the southern entrance on Jackson Boulevard.<ref name="Chicago YIMBY 2020" /> The Wacker Drive lobby contains ''In the Heart of this Infinite Particle of Galactic Dust'', a 2019 artwork by Jacob Hashimoto. It consists of over 7,000 rice-paper and resin disks that are hung from the ceiling.<ref name="Willis Tower Art of the Neighborhood New" /><ref name="Koziarz 20192">{{cite web |last=Koziarz |first=Jay |date=July 16, 2019 |title=Cloud-like sculpture of 7,000 hanging disks floats above Willis Tower lobby |url=https://chicago.curbed.com/2019/7/16/20696661/willis-tower-renovation-art-jacob-hashimoto |access-date=September 7, 2022 |website=Curbed Chicago}}</ref> To honor Khan's contributions to skyscraper engineering design, the Structural Engineers Association of Illinois also commissioned a sculpture of him for the lobby of the Willis Tower.<ref>{{cite web |title=Fazlur Khan Sculpture |url=https://www.seaoi.org/history-past-presidents-khan-sculpture |access-date=February 25, 2014 |publisher=SEAOI}}</ref>

The commercial complex at the building's lowest stories is known as the Catalog, a reference to Sears' [[Mail order|mail-order catalogs]].<ref name="architecturalrecord.com" /><ref name="Selvam 2019">{{cite web |last=Selvam |first=Ashok |date=October 16, 2019 |title=Inside Willis Tower's $500 Million Renovation, Which Includes New Restaurants and a Food Hall |url=https://chicago.eater.com/2019/10/16/20914831/willis-tower-new-restaurants-food-walkthrough-catalog-eq-office-urbanspace-photos-images-food-hall |access-date=September 7, 2022 |website=Eater Chicago}}</ref> The six-story complex includes numerous restaurants.<ref name="Selvam 2019" /> It extends into three of the building's basement levels, as well as the three-story annex to the south and west of the tower.<ref name="architecturalrecord.com" /> The roof of the annex includes a curved [[skylight]] with 240 glass panes, and the northern section of the annex's roof is supported by black columns that resemble those in the original tower. The Catalog also contains decorative details, such as handrails and staircase landings, which are inspired by elements of Chicago's "built environment".<ref name="Koziarz 2019">{{cite web |last=Koziarz |first=Jay |date=October 15, 2019 |title=Checking in on Willis Tower's new retail and dining complex |url=https://chicago.curbed.com/2019/10/15/20914151/willis-tower-renovation-construction-retail-food-hall-catalog |access-date=September 7, 2022 |website=Curbed Chicago}}</ref> The third story of the Catalog contains a 30,000-square-foot [[coworking]] space operated by Convene.<ref name="Chicago YIMBY 2020" /><ref name="Koziarz 2019" />

=== Skydeck ===
[[File:Willis tower skyboxes (1).jpg|thumb|Four glass-bottom skyboxes on the west facade of the Willis Tower at the 103rd floor]]
[[File:Willis Tower glass box.jpg|thumb|left|Glass balcony at the Skydeck]]
<!-- This section is linked from [[John Hancock Center]] -->
The Willis Tower observation deck, called the Skydeck, opened on June 22, 1974. Located on the 103rd floor, {{convert|1353|ft|m|1}} above ground level, it is the highest observation deck in the United States<ref name="ABC7 New York 2013 o073">{{cite web | title=One World Trade Center in New York to surpass Willis Tower in Chicago as tallest building in US | website=ABC7 New York | date=November 12, 2013 | url=https://abc7ny.com/archive/9322666/ | access-date=November 20, 2023}}</ref> and one of Chicago's most famous tourist attractions.<ref>{{cite web |last1=Rackl |first1=Lori |title=Chicago spot makes TripAdvisor's list of world's top tourist attractions in 2019 |url=https://www.chicagotribune.com/travel/ct-trav-most-popular-tourist-attractions-tripadvisor-1216-20191216-c5cjsjufrfdhriwzdvty2cfcge-story.html |publisher=Chicago Tribune |access-date=October 12, 2021 |date=December 15, 2019}}</ref> Tourists can experience how the building sways in wind and see far over the plains of Illinois and across [[Lake Michigan]] to [[Indiana]], [[Michigan]], and [[Wisconsin]] in clear conditions. Elevators reach the top in about 60 seconds, allowing occupants to feel the change in pressure as they ascend. Some 1.7&nbsp;million tourists visit annually {{as of|2018|lc=y}}.<ref name="Lewis 2019 k926">{{cite web | last=Lewis | first=Sophie | title=Protective layer of Willis Tower's SkyDeck cracks under visitors' feet | website=CBS News | date=June 12, 2019 | url=https://www.cbsnews.com/news/willis-towers-skydeck-chicago-cracks-protective-layer-under-visitors-feet-2019-06-12/ | access-date=November 20, 2023}}</ref><ref name="Ori 2018 t377">{{cite web | last=Ori | first=Ryan | title=Willis Tower owner seeks thousands of new visitors per day, as $500 million expansion takes shape | website=Chicago Tribune | date=September 13, 2018 | url=https://www.chicagotribune.com/columns/ryan-ori/ct-biz-willis-tower-expansion-ryan-ori-20180907-story.html | access-date=November 20, 2023}}</ref> There is also an event venue on the 99th floor.<ref name="Skydeck Chicago 2023 v657">{{cite web | title=Chicago Event Venues | website=Skydeck Chicago | date=April 25, 2023 | url=https://theskydeck.com/private-events/ | access-date=November 20, 2023}}</ref>

In January 2009, a major renovation of the Skydeck began, including the installation of retractable glass balconies which extend approximately {{convert|4|ft|m}} from the facade of the 103rd floor, overlooking South Wacker Drive. The all-glass boxes, informally dubbed "The Ledge", allow visitors to see the street below. The boxes, which can accommodate {{convert|5|ST|MT|abbr=off}}, opened to the public on July 2, 2009.<ref>[https://www.usatoday.com/travel/destinations/2009-07-01-sears-tower-glass-balconies_N.htm "Sears Tower unveils 103rd floor glass balconies"], ''USA Today'', July 1, 2009</ref><ref>{{cite web|url=http://www.som.com/content.cfm/sears_tower_observation_deck|title=The Ledge at Skydeck Chicago|access-date=May 15, 2011|archive-url=https://web.archive.org/web/20110608034757/http://www.som.com/content.cfm/sears_tower_observation_deck|archive-date=June 8, 2011|url-status=dead}} SOM.com Project Page</ref> On May 29, 2014, the [[laminated glass]] flooring of one of the boxes cracked while visitors were inside, but there were no injuries.<ref>{{cite web|author=BJ Lutz |url=http://www.nbcchicago.com/news/local/chicago-willis-tower-sky-deck-ledge-crack-261079001.html |title=Coating on Willis Tower Skydeck's Ledge Cracks Under Tourists |publisher=Nbcchicago.com |date=May 30, 2014 |access-date=August 26, 2014}}</ref> The flooring of the same box cracked again on June 12, 2019.<ref>{{cite web|author=Sophie Lewis |url=https://www.cbsnews.com/news/willis-towers-skydeck-chicago-cracks-protective-layer-under-visitors-feet-2019-06-12/ |title=Protective layer of Willis Tower's SkyDeck cracks under visitors' feet |publisher=cbsnews |date=June 12, 2019|access-date=June 12, 2019}}</ref>  In May 2022 a fifth glass ledge opened on the west facade overlooking South Wacker Drive.
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{{Wide image|WillisTowerPanorama01.jpg|2300px|Panorama of Chicago skyline as seen from Willis Tower Skydeck||center|}}