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Conference Session A5

Paper # 3189

University of Pittsburgh 1 April 3, 2013Swanson School of Engineering

THE BUTTRESSED CORE STRUCTURAL SYSTEMJanna Brown (jib26@pitt.edu, Budny 4:00), Jenna Williams (jrw110@pitt.edu, Budny 4:00)

AbstractThe tallest building in the world, the Burj Khalifa,symbolizes a major leap in structural engineering through

its innovated buttressed core structural system. In the 32

years between the completion of one World Trade Center

and Taipei 101, the height of the worlds tallest building had

only been increased by 22 percent. Upon its completion, the

Burj Khalifa, standing at a height of 828 meters, surpassed

Taipei 101 by more than 60 percent [1]. This massive jump

in height can be attributed to the invention of the buttressed

core structural system.

This structural system was first developed in the

Skidmore, Owings, and Merrill (SOM) architectural and

engineering firms design of Tower Palace III in Seoul,

South Korea. Tower Palace III exhibited very goodstructural behavior and performed well in the wind tunnel,

implying to engineers that it could be built much higher [1].

This building, however, could not reach its height potential

because of zoning issues, and so the design was not fully

developed. During the design process for the Burj Khalifa,

engineers altered the Tower Palace III design, allowing for

an even greater maximum height [1].

The Burj Khalifa was designed to be a sustainable

building. Engineers and architects worked together to

reduce the environmental impact of the building and to

minimize its energy consumption. Through a number of

techniques, the Burj Khalifa became a leader in the

sustainable design of skyscrapers.

This paper will explore the buttressed core structural

system of both the Burj Khalifa building and Tower Palace

III and explain how its tripod shape base and stepped

setbacks allow for extreme building height. The stepped

setbacks ability to prevent organization of wind vortexes

will also be explored and explained. Through stability

principles derived from solid mechanics, the effectiveness of

the codependence of the three wings and the central core

will be explained, allowing for an overall more in-depth

understanding of the buttressed core structural system.

Key Words- Burj Khalifa, Tower Palace III, Buttressed,

Hexagonal Hub, Wind Effects, Three Wings

A NEW ERA IN THE DESIGN OF

SKYSCRAPERS

The Tower Palace III introduced the engineering andarchitectural worlds to an entirely new approach to buildingskyscrapers, known as the buttressed core structural system.This structural system then evolved and extended itspotential for incredible building heights in the design andconstruction of the building that currently boasts the title oftallest in the world, the Burj Khalifa. The designers of theBurj Khalifa, a sustainable building, utilized a number of

techniques to reduce the buildings energy consumption. Themuch-anticipated Kingdom Tower will also utilize thebuttressed core structural system to climb to a height of over1,000 meters (exceeding the Burj Khalifa by more than 100meters).

The crux of the buttressed core structural system is itstripod-shaped design featuring a sturdy central coresurrounded by three building wings. In this system, thewings are codependent and each is supported (buttressed) bythe other two [1]. The torsional resistance for the building issupplied by the strong, six-sided central core (or hexagonalhub). The three wings afford the shear resistance andincrease the moment of inertia, and as the building rises,

each wing sets back in a clockwise pattern [2]. This taperingas the building rises is necessary to minimize the windeffects and prevent the organization of wind vortices overthe height of the tower [1]. The give-and-take between thecore of the building and its wings are the key to thestructural system and allow for taller, more stableskyscrapers.

TOWER PALACE

Skidmore, Owings, and Merrill (SOM), a prestigiousarchitectural and engineering firm based in Chicago, Illinois,designed the buttressed core structural system for both the

Tower Palace III and the Burj Khalifa. The firms practice ofhaving architects and engineers work together closely onprojects seems to have assisted in the conception of many ofthe firms greatest creations, including the Willis Tower(formerly known as the Sears Roebuck Tower). WilliamBaker, the head structural engineer in SOM is recognized asthe main engineer behind the creation of the buttressed corestructural system [3].

The Tower Palace III, completed in 2004, was originallyplanned to be a 320 meter, all-residential building in theKangnam district of Seoul, South Korea [4]. When SOMundertook the project, the architects and engineers werefaced with the challenge of controlling the dynamicresponse of the tower and managing its wind engineeringaspects [4]. The design team drafted three differentschemes for the building with the same total floor area andsimilar number of apartment units. The third scheme, whichwas the shortest of the three options, was eventually chosenas the final design.

SOM created the Tower Palace III based on a set ofgoals. These goals include: optimiz[ing] [the] towerstructural system for strength [and] stiffness, using gravityloads to resist lateral loads, and limiting the torsion on thebuilding [4]. These goals were accomplished through the y-shaped structural system, which was designed to maximizeviews from the tower and for the intake of natural light.

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Engineers and architects then discovered that this shape wasincredibly stable and strong [4].

Limitations of the Tower Palace III

Upon completion, the Tower Palace III became the tallestbuilding in South Korea, but it did not fulfill its heightpotential. Strict zoning issues in Seoul prevented SOM fromdesigning the 93 story building that had once beenenvisioned (the Tower Palace now stands at 73 stories tall).Local residents and authorities also expressed concerns overthe buildings height and possible traffic congestion [4].Despite the Tower Palace IIIs solid structural behavior,SOM architects and engineers encountered issues with thebuildings torsional resistance. This lack of torsionalresistance means that, as the building grows in height, it willbegin to twist along its vertical axis. Baker identified this asa major problem in the design of skyscrapers and sought toinvent a solution.

BURJ KHALIFA

The idea for the tallest structure ever constructed in thehistory of mankind came to William Baker while he wasworking with SOM. The difficulties and challenges that arisewhile designing and building the tallest building in the worlddemand that architects and engineers collaborate to pushcurrent analytical, material, and construction technologiesto new heights [5]. Architects and engineers workedtogether to alter orthodox systems, resources, and buildingmethods to create the Burj Khalifa in Dubai. Bakers goal ofthe project was to design a building that reached greatheights without consuming a large volume of space whilealso resist[ing] the forces of nature in a simple way [6]. Healso was responsible for meeting owner Emaar PropertiesPublic Joint Stock Companys expectations. The BurjKhalifa needed to have enough width to support itself and tobe narrow enough to create economically viable real estatefor the client [6].

The Burj Khalifa is the focal point of a large developmentalso containing a low-rise office annex, a two-story poolannex, and an adjacent podium structure. The tower itselfserves mostly residential and office purposes, but also

contains retail stores and a Giorgio Armani hotel. The $1.5billion structure holds the title of tallest building in the worldin three categories measured by the Council on TallBuildings and Urban Habitat. These categories include:height to tip, height to architectural top, and height tohighest occupied floor. The Burj Khalifa measures 829.8meters to tip, 828 meters to architectural top, and 584.5meters to highest occupied floor. It claimed these records bybeating out the Willis Tower (527 meters), Taipei 101 (508meters), and Shanghai World Financial Center (474 meters),respectively [7]. The record-shattering height of the BurjKhalifa can be largely credited to its use of the buttressedcore structural system featuring high-performance concrete

wall construction with a hexagonal hub and three buttressedwings [5].

Hexagonal Hub

Perhaps the most crucial aspect of the buttressed corestructural system is its six-sided center piece. This featurenot only provides torsional resistance and prevents twistingof the tower, it acts as an axle that encloses the elevators[6]. The central core allows for torsional resistance throughcorridor walls built of high performance concrete thatextends from the core down the axis of each wing. Thesecorridor walls strategically end in thickened hammerheadwalls which lie perpendicular to length of the corridor walls.The closed hexagonal core, a unique feature of thebuttressed core system, acts like a tube surrounding thebuilding and helps to make it torsionally stiff.

As buildings get taller, they become more susceptible totwisting about their vertical axis. The buttressed core systemsolves this problem by using the three building wings tobuttress (support) the center core, with the center core in turnallowing the wings to be supported by each other. Thesewings make it harder for the entire building to twist about itsvertical axis.

Thickened hammerhead walls located at the end of thecorridors running down through the wings also prevent thebuilding from twisting about its vertical axis (providing itwith torsional stiffness) because of moments of inertia. Alarge amount of concrete placed this far away from thecenter of the structure results in large moments of inertia.

This means that the structure not only has large torsionalstiffness but that it also has a very large lateral bendingstiffness to resist bending effects from lateral loads (such aswind). Figure 1 displays the cross section at the base of theWillis Tower, a hypothetical Willis Tower II (showing thenecessary base of the Willis Tower if it were as tall as theBurj Khalifa), and the Burj Khalifa.

FIGURE 1

Figure 1a. Comparison of all three floor plans [8]

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Figure 1b. Comparison of Hypothetical Willis Tower andBurj Dubai [8]

Figure 1b displays the difference in the size of the floorplan of the hypothetical Willis Tower and the Burj Dubai.The dark grey represents useable floor space while the light

grey represents unuseable floor space. Clearly, the BurjDubai has a much greater useable to unuseable space ratiothan the hypothetical Willis Tower.

Figure 1c. Comparison of Real Willis Tower and Burj Dubai[8]

Figure 1c shows the standing Willis Tower with muchmore useable space than Figure 1b. Although this buildinghas much more useable space, it can only reach a smallermaximum height. The red elements, representing thestructural system, clearly highlight the corridor walls liningeach wing and ending in thickened hammerhead walls.

The design that SOM created also minimizes the effectsof differential shortening (shrinkage), which is a major

consideration for very tall buildings. The design teamaddressed this issue by changing wall thickness and columnsizes on select features of the Burj Khalifa. Outrigger wallsscattered up the building provide equal gravity loadsthroughout the building, minimizing differential creepmovements [9]. Because shrinkage occurs more quickly inthinner walls and columns, the perimeter column thicknessmimics the typical corridor wall thickness. The thickness ofthese perimeter columns is determined by stress on theinterior corridor walls [9]. Figure 2 is a simple floor plan ofthe Burj Khalifa showing the three wings, the hexagonalhub, the corridor walls, and the hammerhead walls.

FIGURE 2

Figure 2. Comparison of floor plans [10]

Overall, the building was designed with differentthicknesses and column sizes such that the concrete wouldshrink uniformly throughout the building without distortingthe shape of the tower.

Necessity of Three Wings

The three wings of the Burj Khalifa allow for greater

building height by buttressing one another via the centralcore (hence the name buttressed core structural system).The wings support the core against lateral loads, and as theheight of the building increases, one wing on each tier setsback in a spiraling pattern, emphasizing the height of thetower. These setbacks are also aesthetically pleasing foroccupants of the tower because they maximize natural lightand the number of rooms with views. The wings wereconstructed such that the perimeter columns on each floorlined up with the walls below them, providing a smooth loadpath [5]. Setbacks usually require transfer beams to passgravity loads from floor to floor, but the Burj Khalifageometry allows for column loads to betransferred directly

to the walls below without transfer beams, which ultimatelyresults in a more efficient building.

Throughout the Burj Khalifa, five mechanical floors arestrategically placed about 30 floors apart. On each of thesemechanical floors, outrigger walls attach the perimetercolumns with the interior wall system. This allows theperimeter columns to contribute to the lateral load resistance,permitting all of the vertically placed concrete to participatein resisting both gravity and lateral loads [5]. Theseoutrigger walls are only placed on the mechanical floorsbecause they would interfere with the usage of functionalfloors.

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Wind at High Heights

One of the biggest obstacles facing structural engineers in

the design of skyscrapers is wind. For very tall and slenderstructures, such as the Burj Khalifa, two major influences onthe structural design are the forces of wind and the motioncaused by these forces [9]. Architects and engineers wereaware that building a tower of great height such as the BurjKhalifa would require understanding, taming, and workingwith the forces of nature [6]. Wind tunnel models wereused to account for the cross wind effects of wind inducedvortex shedding on the building [9]. Some of the windtunnel tests, such as the aeroelastic and force balancestudies, were done with models at a scale of 1:500 (althoughthe pedestrian wind tests also used a model of scale 1:250)[5]. Despite the design teams awareness of the challengespresented by wind at such great heights, the first wind tunnelresults for the Burj Khalifa were poor. This was, in part, dueto an overestimation of the wind climate but mostly due tolack of aerodynamic behavior by the building. After each setof wind tunnel testing, the design team altered the shape ofthe tower to confuse the wind and minimize the effects ofvortex shedding on the building [9]. Setbacks wereorganized to change the towers width at each setback. Thisprevents the wind vortices from becoming organizedbecause the building is constantly changing shape. Thedesign team also used gravity to counter the wind forcessimilar to the way one would spread his/her legs in a strongwind for stability.

Liquefaction and Seismic Considerations

Seismic activity is always a major concern in theconstruction of skyscrapers. In the Uniform Building Code,Dubai is classified as zone 2a (moderate seismic activity).This means that Dubais seismic activity is comparable tothat of New York City and Boston [5]. Because of this lowclassification, seismic activity did not have a large effect onthe reinforced-concrete tower design, but it did direct thedesign of the steel spire structure at the top of the BurjKhalifa which holds the communications and mechanicalfloors. Soil liquefaction is also a potential issue with theconstruction of skyscrapers. Soil liquefaction occurs when

an applied stress causes solid soil to temporarily behave as aviscous liquid. However, when potential of soil liquefactionin the area was examined, it was deemed structurallyirrelevant for the buildings deep-rooted foundations [5].

A Leader in Sustainable Design

A sustainable building has the capacity to be maintainedfor a long period of time. The Burj Khalifa was constructedwith the future in mind. It is remarkable not only because ofits height, but also because of its integration of sustainabledesign. The building employs many different energy and

cost saving methods to remain sustainable and moreenvironmentally friendly.

The design team for the Burj Khalifa made extensive

efforts to address the high energy consumption that isusually associated with skyscrapers and cities. Currently,urban areas account for about sixty percent of the worldsenergy consumption [11]. To minimize unnecessary energyconsumption, the Burj Khalifa utilizes a special buildingmanagement system with smart lighting and mechanicalcontrol [2]. This system, created by Asea Brown Boveri,Ltd., uses computer based systems to monitor and controlelectricity [11]. The resulting effect is a more efficient useof energy and a smaller environmental impact.

To fulfill the water heating needs of the buildingsresidents, the Burj Khalifa utilizes solar power. 378 collectorpanels, each with an area of 2.7 square meters, lie on theroof of the office annexes. These panels have the ability toheat 140,000 liters of water when supplied with just sevenhours of daylight. This is equivalent to 32,000 kilo watts ofenergy per day [12].

The building also employs other water-related sustainablepractices. The Burj Khalifa uses a massive condensaterecovery system, one of the largest in the world [13]. Thiscondensate recovery system collects water condensate fromthe air conditioning system and diverts it to an irrigationtank located on-site. This prevents the condensate dischargefrom becoming waste water and, in total, provides about 15million gallons of supplemental water per year [12]. Thewater collected is used for irrigation of the landscape aroundthe Burj Khalifa and is enough to fill 14 Olympic sized

swimming pools [13]. This condensate recovery systemreuses millions of gallons of water each year, lowering thewater-related expenses of the building and making it moreenvironmentally friendly.

The air conditions at the top of the Burj Khalifa allow forreduced energy consumption as well. Sky sourcedventilation uses air ventilation at the top of the building toreduce the amount of energy consumed by air conditioning,ventilation, and dehumidification. The air drawn in at the topof the building is cooler and has a lower density and relativehumidity than the air at the bottom of the Burj Khalifa [13].These conditions are ideal for ventilation of buildings, andso less energy is required to maintain comfortable conditions

within the building.Because of its sustainable design, the Burj Khalifa has

lowered its energy consumption impact on the world and ismore environmentally friendly than a lot of otherskyscrapers. However, super tall buildings, such as the BurjKhalifa, still have a huge impact on the environment, and sosustainable design will continue to be a major factor in thefuture design of these buildings.

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CONTROVERSIAL ETHICS AND

DISADVANTAGES OF SKYSCRAPERS

As buildings grow in size, so do the number of ethicalcontroversies that accompany this size. Higher buildingstypically require larger bases. Bases for skyscrapers (whichtypically stand in cities) require large plots of land and causethe destruction of the neighboring urban fabric [10]. Thesestructures also darken cities by casting large shadows andmaking sunlight less accessible at street level.

Perhaps the most pressing ethical controversy stemmingfrom skyscrapers is the safety of the people inside of them.Very limited safety protocols can be made for a building astall as the Burj Khalifa. Is it practical to expect a timely andcalm evacuation from the top floor of a mile-high building inthe case of a fire? An evacuation plan more efficient than

calmly using the stairs needs to be developed for skyscrapersso that the lives of the residents and occupants of thesebuildings are no longer at great risk. The first canon of theCivil Engineering Code of Ethics states that engineers shallhold paramount the safety, health, and welfare of the publicand shall strive to comply with the principles of sustainabledevelopment in the performance of their professional duties[14]. The question for engineers is no longer how high can abuilding be constructed, but how high can it be constructedsafelyfor its occupants?

Since the Twin Towers fell on September 11, 2001 inNew York City, there has been an even greater stigmasurrounding the topic of skyscrapers. Because of their largenumber of occupants and often iconic status, skyscrapers canbe targets for terrorist attacks. The events of September 11,2001 directly affected SOM itself by preventing a kickoffmeeting for a 160 story building (which would have becomethe tallest building in the world at that time). The projectwas postponed and then altered to reach a smaller maximumheight of only 92 stories [3].

With taller buildings also come much higher prices.Construction costs of skyscrapers increase exponentially asthe building grows in height. Baker estimates that for abuilding that has the same footprint but twice as high, thecost of every square foot becomes somewhere between fourand eight times as much [3]. A major issue with tallerskyscrapers is transportation. More floors mean longer waits

for elevators and longer elevator shafts. More effectivetransportation systems in skyscrapers need to be developedto address this issue.

THE FUTURE OF THE BUTTRESSED

CORE STRUCTURAL SYSTEM

SOM and Baker made history with the innovation of thebuttressed core structural system, and the competition tobuild the tallest building in the world continues. The idea ofa central core and three wings revolutionized the way thatskyscrapers are structured and altered the approach that

many engineers take when designing a building. AdrianSmith, an architect and former Design Partner at SOM,worked closely with Baker on the Burj Khalifa. Smith is one

of the architects behind what is expected to become theworlds tallest building in 2018 [15].In 2009, Prince Alweed bin Talal of the Saudi royal

family invited eight design firms to submit designs for thetallest building in the world. The aim for the design was torepresent Saudi Arabia as a global icon. The submission bySmith and his colleague at Adrian Smith + Gordon GillArchitecture (AS+GG) was chosen as the winner of thecompetition. The Kingdom Tower, to be located in Jeddah,Saudi Arabia, is expected to be over 1,000 meters tall (172meters taller than the Burj Khalifa). The skyscraper willstand at the heart of a 57 million square foot developmentand will contain a Four Seasons Hotel, apartments, officespace, and the worlds highest observatory [15].

The Kingdom Tower shares the same buttressed corestructural system with the Burj Khalifa, but architects andengineers made alterations to the design to accommodate forheight, wind climate, and the clients wishes. The wings ofthe Kingdom Tower will not setback in the way that thewings of the Burj Khalifa do. The Kingdom Towers wingswill be tapered rather than stepped as they ascend towardthe sky [15]. For a more dynamic appearance, each willterminate at a different angle.

Like the engineers and architects at SOM during thedesign process for the Burj Khalifa, the design team for theKingdom Tower focused on minimizing the effects of windon the skyscraper. Because of the structures unique shape,

the structural engineers on the project are working with thewind consultant to conduct extensive wind tunnel tests onthe building [15]. Engineers believe that the concavecurvature of the sides of the Kingdom Tower will help toalleviate the effects of wind on the skyscraper.

CONCLUSION

Beginning with the Tower Palace III, then expanding itspotential with the Burj Khalifa, and now reaching evengreater heights through the Kingdom Tower, the buttressedcore structural system has forever altered the design ofskyscrapers. Sustainable design, such as that seen in the Burj

Khalifa, must continue to be used to make skyscrapers moreenvironmentally friendly and less energy consuming.

From 1972 to 2004, the world saw only a 22 percentincrease in the height of the worlds tallest building. Uponits inauguration on January 4, 2010, the Burj Khalifa becamethe tallest building in the world (surpassing the previous titleholder by over 60 percent). This massive jump in buildingheight cannot be overlooked by the engineering community.Bakers y-shaped structural system is the future of designingskyscrapers and may be the key to reaching unfathomablebuilding heights. The buttressed core structural system has,without a doubt, revolutionized the structure and design ofskyscrapers throughout the world.

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REFERENCES

[1] W. F. Baker. (2010). Higher and Higher: The Evolution

of the Buttressed Core. Civil Engineering. (Print Article).pp. 58-65.[2] World Buildings Directory. Buttressed Core StructuralSystem for Burj Khalifa. (Online Article).http://www.worldbuildingsdirectory.com/project.cfm?id=2618[3] Blum, Andrew. "Engineer Bill Baker Is the King ofSuperstable 150-Story Structures." Wired Magazine27 Nov.2007: n. pag. Web.[4] Abdelrazaq, Baker, Chung, Pawlikowski, Wang, andYom.Integration of Design and Construction of the TallestBuilding in Korea, Tower Palace III, Seoul, Korea. 10 Oct.2004. South Korea, Seoul.

[5] Baker, William, James Pawlikowski, and BradleyYoung. "Reaching toward The Heavens."CivilEngineeringMar. 2010.[6] Baker, William. "Engineering an Idea: The Realizationof the Burj Khalifa." Civil Engineering.[7] "Burj Khalifa Facts." Skyscrapercenter. Council on TallBuildings and Urban Habita, n.d. Web. 07 Mar. 2013.[8] Bollinger, Peter. The Buttressed Core. Digitalimage. Wired Magazine. N.p., 27 Nov. 2007[9] Baker, William, Stanton Korista, and Lawrence Novak."Engineering the World's Tallest - Burj Dubai." Council onTall Buildings and Urban Habitat(2008)[10]Burj Khalifa Typical Floor Plan. Digital image.AccessScience. Silver Chair, 2010. Web.[11] Helms, Jeremy. "Header Menu."Industry Tap. N.p.,2011. Web.

[12]"LexisNexis Academic & LibrarySolutions."LexisNexis Academic & Library Solutions.Emirates News Agency (WAM), 4 Apr. 2010. Web.

[13] "Burj Dubai, the Shining Building." GUARDIANGlass, 2010. Pdf.[14]"Code of Ethics."American Society of Civil Engineers.N.p., n.d. Web. 04 Mar. 2013[15] Jones, Jenny. Worlds Tallest Building Must Be MoreTall. Civil Engineering (08857024)81.9(2011):16-17.Military & Government Collection.

ADDITIONAL REFERENCESBlum, Andrew. "Engineer Bill Baker Is the King ofSuperstable 150-Story Structures." Wired Magazine27 Nov.2007: n. pag. Web."Design."Burj Khalifa RSS. InternetCont Ltd., n.d.

Lowe, Aya, and Samia Badih. "Burj Khalifa Is anOutstanding Example of Sustainable Architecture."LatestNews, Pictures, Video, Multimedia. N.p., 6 Jan. 2010. Web."Skidmore, Owings & Merrill." International Directory ofCompany Histories. Ed. Tina Grant. Vol. 13. Detroit: St.James Press, 1996. 475-476. Gale Virtual Reference Library.Stephens, Suzanne. Kingdom Come.Architectural Record200.5(2012):160 Business Source Complete. Web. 4 Mar.2013

ACKNOWLEDGEMENTS

We would like the thank our Chairs, John Broscious andBenjamin Hunter, our Co- Chair, Agatha Carlin, and thewriting center (particularly Nancy Koerbel, Lauren Gattos,and Jillian Harkins) as well as the library staff for theirresources and assistance.