Buttressed Core 72uo

Conference Session A5 Paper # 3189

THE BUTTRESSED CORE STRUCTURAL SYSTEM Janna Brown ([email protected], Budny 4:00) Abstract—The 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 world’s tallest building had only been increased by 22 percent. Upon its completion, the Burj Khalifa, standing at a height of 828 meters, sured 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 firm’s design of Tower Palace III in Seoul, South Korea. Tower Palace III exhibited very good structural 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 and architectural worlds to an entirely new approach to building skyscrapers, known as the buttressed core structural system. This structural system then evolved and extended its potential for incredible building heights in the design and construction of the building that currently boasts the title of tallest in the world, the Burj Khalifa. The designers of the Burj Khalifa, a sustainable building, utilized a number of University of Pittsburgh Swanson School of Engineering

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techniques to reduce the building’s energy consumption. The much-anticipated Kingdom Tower will also utilize the buttressed core structural system to climb to a height of over 1,000 meters (exceeding the Burj Khalifa by more than 100 meters). The crux of the buttressed core structural system is its tripod-shaped design featuring a sturdy central core surrounded by three building wings. In this system, the wings are codependent and each is ed (buttressed) by the other two [1]. The torsional resistance for the building is supplied by the strong, six-sided central core (or hexagonal hub). The three wings afford the shear resistance and increase the moment of inertia, and as the building rises, each wing sets back in a clockwise pattern [2]. This tapering as the building rises is necessary to “minimize the wind effects” and prevent the organization of wind vortices over the height of the tower [1]. The give-and-take between the core of the building and its wings are the key to the structural system and allow for taller, more stable skyscrapers.

TOWER PALACE Skidmore, Owings, and Merrill (SOM), a prestigious architectural and engineering firm based in Chicago, Illinois, designed the buttressed core structural system for both the Tower Palace III and the Burj Khalifa. The firm’s practice of having architects and engineers work together closely on projects seems to have assisted in the conception of many of the firm’s greatest creations, including the Willis Tower (formerly known as the Sears Roebuck Tower). William Baker, the head structural engineer in SOM is recognized as the main engineer behind the creation of the buttressed core structural system [3]. The Tower Palace III, completed in 2004, was originally planned to be a 320 meter, all-residential building in the Kangnam district of Seoul, South Korea [4]. When SOM undertook the project, the architects and engineers were faced with the challenge of “controlling the dynamic response of the tower and managing its wind engineering aspects” [4]. The design team drafted three different schemes for the building with the same total floor area and similar number of apartment units. The third scheme, which was the shortest of the three options, was eventually chosen as the final design. SOM created the Tower Palace III based on a set of goals. These goals include: “optimiz[ing] [the] tower structural system for strength [and] stiffness,” using gravity loads to resist lateral loads, and limiting the torsion on the building [4]. These goals were accomplished through the yshaped structural system, which was designed to maximize views from the tower and for the intake of natural light. April 3, 2013

Janna Brown Jenna Williams Engineers and architects then discovered that this shape was incredibly stable and strong [4].

wall construction” with a hexagonal hub and three buttressed wings [5].

Limitations of the Tower Palace III

Hexagonal Hub

Upon completion, the Tower Palace III became the tallest building in South Korea, but it did not fulfill its height potential. Strict zoning issues in Seoul prevented SOM from deg the 93 story building that had once been envisioned (the Tower Palace now stands at 73 stories tall). Local residents and authorities also expressed concerns over the building’s height and possible traffic congestion [4]. Despite the Tower Palace III’s solid structural behavior, SOM architects and engineers encountered issues with the building’s torsional resistance. This lack of torsional resistance means that, as the building grows in height, it will begin to twist along its vertical axis. Baker identified this as a major problem in the design of skyscrapers and sought to invent a solution.

Perhaps the most crucial aspect of the buttressed core structural system is its six-sided center piece. This feature not only provides torsional resistance and prevents twisting of the tower, it “acts as an axle that encloses the elevators” [6]. The central core allows for torsional resistance through corridor walls built of high performance concrete that extends from the core down the axis of each wing. These corridor walls strategically end in thickened hammerhead walls which lie perpendicular to length of the corridor walls. The closed hexagonal core, a unique feature of the buttressed core system, acts like a tube surrounding the building and helps to make it torsionally stiff. As buildings get taller, they become more susceptible to twisting about their vertical axis. The buttressed core system solves this problem by using the three building wings to buttress () the center core, with the center core in turn allowing the wings to be ed by each other. These wings make it harder for the entire building to twist about its vertical axis. Thickened hammerhead walls located at the end of the corridors running down through the wings also prevent the building from twisting about its vertical axis (providing it with torsional stiffness) because of moments of inertia. A large amount of concrete placed this far away from the center of the structure results in large moments of inertia. This means that the structure not only has large torsional stiffness but that it also has a very large lateral bending stiffness to resist bending effects from lateral loads (such as wind). Figure 1 displays the cross section at the base of the Willis Tower, a hypothetical Willis Tower II (showing the necessary base of the Willis Tower if it were as tall as the Burj Khalifa), and the Burj Khalifa.

BURJ KHALIFA The idea for the tallest structure ever constructed in the history of mankind came to William Baker while he was working with SOM. The difficulties and challenges that arise while deg and building the tallest building in the world demand that architects and engineers collaborate to push “current analytical, material, and construction technologies to new heights” [5]. Architects and engineers worked together to alter orthodox systems, resources, and building methods to create the Burj Khalifa in Dubai. Baker’s goal of the project was to design a building that reached great heights without consuming a large volume of space while also “resist[ing] the forces of nature in a simple way” [6]. He also was responsible for meeting owner Emaar Properties Public t Stock Company’s expectations. The Burj Khalifa needed to have enough width to itself and to be narrow enough to “create economically viable real estate for the client” [6]. The Burj Khalifa is the focal point of a large development also containing a low-rise office annex, a two-story pool annex, and an adjacent podium structure. The tower itself serves mostly residential and office purposes, but also contains retail stores and a Giorgio Armani hotel. The $1.5 billion structure holds the title of tallest building in the world in three categories measured by the Council on Tall Buildings and Urban Habitat. These categories include: height to tip, height to architectural top, and height to highest occupied floor. The Burj Khalifa measures 829.8 meters to tip, 828 meters to architectural top, and 584.5 meters to highest occupied floor. It claimed these records by beating out the Willis Tower (527 meters), Taipei 101 (508 meters), and Shanghai World Financial Center (474 meters), respectively [7]. The record-shattering height of the Burj Khalifa can be largely credited to its use of the buttressed core structural system “featuring high-performance concrete

FIGURE 1

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

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Janna Brown Jenna Williams FIGURE 2

Figure 1b. Comparison of Hypothetical Willis Tower and Burj Dubai [8] Figure 1b displays the difference in the size of the floor plan 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 Burj Dubai has a much greater useable to unuseable space ratio than the hypothetical Willis Tower. Figure 2. Comparison of floor plans [10] Overall, the building was designed with different thicknesses and column sizes such that the concrete would shrink uniformly throughout the building without distorting the 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 central core (hence the name “buttressed core structural system”). The wings the core against lateral loads, and as the height of the building increases, one wing on each tier sets back in a spiraling pattern, emphasizing the height of the tower. These setbacks are also aesthetically pleasing for occupants of the tower because they maximize natural light and the number of rooms with views. The wings were constructed such that the perimeter columns on each floor lined up with the walls below them, providing a smooth load path [5]. Setbacks usually require transfer beams to gravity loads from floor to floor, but the Burj Khalifa geometry allows for column loads to be transferred directly to the walls below without transfer beams, which ultimately results in a more efficient building. Throughout the Burj Khalifa, five mechanical floors are strategically placed about 30 floors apart. On each of these mechanical floors, outrigger walls attach the perimeter columns with the interior wall system. This allows the perimeter columns to contribute to the lateral load resistance, permitting all of the vertically placed concrete to participate in resisting both gravity and lateral loads [5]. These outrigger walls are only placed on the mechanical floors because they would interfere with the usage of functional floors.

Figure 1c. Comparison of Real Willis Tower and Burj Dubai [8] Figure 1c shows the standing Willis Tower with much more useable space than Figure 1b. Although this building has much more useable space, it can only reach a smaller maximum height. The red elements, representing the structural system, clearly highlight the corridor walls lining each wing and ending in thickened hammerhead walls. The design that SOM created also minimizes the effects of differential shortening (shrinkage), which is a major consideration for very tall buildings. The design team addressed this issue by changing wall thickness and column sizes on select features of the Burj Khalifa. Outrigger walls scattered up the building provide equal gravity loads throughout the building, minimizing differential creep movements [9]. Because shrinkage occurs more quickly in thinner walls and columns, the perimeter column thickness mimics the typical corridor wall thickness. The thickness of these perimeter columns is determined by stress on the interior corridor walls [9]. Figure 2 is a simple floor plan of the Burj Khalifa showing the three wings, the hexagonal hub, the corridor walls, and the hammerhead walls.

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Janna Brown Jenna Williams cost saving methods to remain sustainable and more environmentally friendly. The design team for the Burj Khalifa made extensive efforts to address the high energy consumption that is usually associated with skyscrapers and cities. Currently, urban areas for about sixty percent of the world’s energy consumption [11]. To minimize unnecessary energy consumption, the Burj Khalifa utilizes a special building management system with “smart lighting and mechanical control” [2]. This system, created by Asea Brown Boveri, Ltd., uses computer based systems to monitor and control electricity [11]. The resulting effect is a more efficient use of energy and a smaller environmental impact. To fulfill the water heating needs of the building’s residents, the Burj Khalifa utilizes solar power. 378 collector s, each with an area of 2.7 square meters, lie on the roof of the office annexes. These s have the ability to heat 140,000 liters of water when supplied with just seven hours of daylight. This is equivalent to 32,000 kilo watts of energy per day [12]. The building also employs other water-related sustainable practices. The Burj Khalifa uses a massive condensate recovery system, one of the largest in the world [13]. This condensate recovery system collects water condensate from the air conditioning system and diverts it to an irrigation tank located on-site. This prevents the condensate discharge from becoming waste water and, in total, provides about 15 million gallons of supplemental water per year [12]. The water collected is used for irrigation of the landscape around the Burj Khalifa and is enough to fill 14 Olympic sized swimming pools [13]. This condensate recovery system reuses millions of gallons of water each year, lowering the water-related expenses of the building and making it more environmentally friendly. The air conditions at the top of the Burj Khalifa allow for reduced energy consumption as well. Sky sourced ventilation uses air ventilation at the top of the building to reduce the amount of energy consumed by air conditioning, ventilation, and dehumidification. The air drawn in at the top of the building is cooler and has a lower density and relative humidity than the air at the bottom of the Burj Khalifa [13]. These conditions are ideal for ventilation of buildings, and so 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 is more environmentally friendly than a lot of other skyscrapers. However, super tall buildings, such as the Burj Khalifa, still have a huge impact on the environment, and so sustainable design will continue to be a major factor in the future design of these buildings.

Wind at High Heights One of the biggest obstacles facing structural engineers in the design of skyscrapers is wind. For very tall and slender structures, such as the Burj Khalifa, two major influences on the structural design are the forces of wind and the motion caused by these forces [9]. Architects and engineers were aware that building a tower of great height such as the Burj Khalifa would require “understanding, taming, and working with the forces of nature” [6]. Wind tunnel models were used to “ for the cross wind effects of wind induced vortex shedding on the building” [9]. Some of the wind tunnel tests, such as the aeroelastic and force balance studies, were done with models at a scale of 1:500 (although the pedestrian wind tests also used a model of scale 1:250) [5]. Despite the design team’s awareness of the challenges presented by wind at such great heights, the first wind tunnel results for the Burj Khalifa were poor. This was, in part, due to an overestimation of the wind climate but mostly due to lack of aerodynamic behavior by the building. After each set of wind tunnel testing, the design team altered the shape of the tower to “confuse the wind” and minimize the effects of vortex shedding on the building [9]. Setbacks were organized to change the tower’s width at each setback. This prevents the wind vortices from becoming organized because the building is constantly changing shape. The design team also used gravity to counter the wind forces similar to the way one would spread his/her legs in a strong wind for stability. Liquefaction and Seismic Considerations Seismic activity is always a major concern in the construction of skyscrapers. In the Uniform Building Code, Dubai is classified as zone 2a (moderate seismic activity). This means that Dubai’s seismic activity is comparable to that of New York City and Boston [5]. Because of this low classification, seismic activity did not have a large effect on the reinforced-concrete tower design, but it did direct the design of the steel spire structure at the top of the Burj Khalifa which holds the communications and mechanical floors. Soil liquefaction is also a potential issue with the construction of skyscrapers. Soil liquefaction occurs when an applied stress causes solid soil to temporarily behave as a viscous liquid. However, when potential of soil liquefaction in the area was examined, it was deemed structurally irrelevant for the building’s deep-rooted foundations [5]. A Leader in Sustainable Design A sustainable building has the capacity to be maintained for a long period of time. The Burj Khalifa was constructed with the future in mind. It is remarkable not only because of its height, but also because of its integration of sustainable design. The building employs many different energy and

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Janna Brown Jenna Williams many engineers take when deg a building. Adrian Smith, 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 the world’s tallest building in 2018 [15]. In 2009, Prince Alweed bin Talal of the Saudi royal family invited eight design firms to submit designs for the tallest building in the world. The aim for the design was to represent Saudi Arabia as a global icon. The submission by Smith and his colleague at Adrian Smith + Gordon Gill Architecture (AS+GG) was chosen as the winner of the competition. The Kingdom Tower, to be located in Jeddah, Saudi Arabia, is expected to be over 1,000 meters tall (172 meters taller than the Burj Khalifa). The skyscraper will stand at the heart of a 57 million square foot development and will contain a Four Seasons Hotel, apartments, office space, and the world’s highest observatory [15]. The Kingdom Tower shares the same buttressed core structural system with the Burj Khalifa, but architects and engineers made alterations to the design to accommodate for height, wind climate, and the client’s wishes. The wings of the Kingdom Tower will not setback in the way that the wings of the Burj Khalifa do. The Kingdom Tower’s wings will be “tapered rather than stepped as they ascend toward the sky” [15]. For a more dynamic appearance, each will terminate at a different angle. Like the engineers and architects at SOM during the design process for the Burj Khalifa, the design team for the Kingdom Tower focused on minimizing the effects of wind on the skyscraper. Because of the structure’s unique shape, the structural engineers on the project are working with the wind consultant to conduct “extensive wind tunnel tests on the building” [15]. Engineers believe that the concave curvature of the sides of the Kingdom Tower will help to alleviate the effects of wind on the skyscraper.

CONTROVERSIAL ETHICS AND DISADVANTAGES OF SKYSCRAPERS As buildings grow in size, so do the number of ethical controversies that accompany this size. Higher buildings typically require larger bases. Bases for skyscrapers (which typically stand in cities) require large plots of land and cause the destruction of “the neighboring urban fabric” [10]. These structures also darken cities by casting large shadows and making sunlight less accessible at street level. Perhaps the most pressing ethical controversy stemming from skyscrapers is the safety of the people inside of them. Very limited safety protocols can be made for a building as tall as the Burj Khalifa. Is it practical to expect a timely and calm evacuation from the top floor of a mile-high building in the case of a fire? An evacuation plan more efficient than calmly using the stairs needs to be developed for skyscrapers so that the lives of the residents and occupants of these buildings are no longer at great risk. The first canon of the Civil Engineering Code of Ethics states that “engineers shall hold paramount the safety, health, and welfare of the public and shall strive to comply with the principles of sustainable development in the performance of their professional duties” [14]. The question for engineers is no longer how high can a building be constructed, but how high can it be constructed safely for its occupants? Since the Twin Towers fell on September 11, 2001 in New York City, there has been an even greater stigma surrounding the topic of skyscrapers. Because of their large number of occupants and often iconic status, skyscrapers can be targets for terrorist attacks. The events of September 11, 2001 directly affected SOM itself by preventing a kickoff meeting for a 160 story building (which would have become the tallest building in the world at that time). The project was postponed and then altered to reach a smaller maximum height of only 92 stories [3]. With taller buildings also come much higher prices. Construction costs of skyscrapers increase exponentially as the building grows in height. Baker estimates that for a building that has the same footprint but twice as high, “the cost of every square foot becomes somewhere between four and eight times as much” [3]. A major issue with taller skyscrapers is transportation. More floors mean longer waits for elevators and longer elevator shafts. More effective transportation systems in skyscrapers need to be developed to address this issue.

CONCLUSION Beginning with the Tower Palace III, then expanding its potential with the Burj Khalifa, and now reaching even greater heights through the Kingdom Tower, the buttressed core structural system has forever altered the design of skyscrapers. Sustainable design, such as that seen in the Burj Khalifa, must continue to be used to make skyscrapers more environmentally friendly and less energy consuming. From 1972 to 2004, the world saw only a 22 percent increase in the height of the world’s tallest building. Upon its inauguration on January 4, 2010, the Burj Khalifa became the tallest building in the world (suring the previous title holder by over 60 percent). This massive jump in building height cannot be overlooked by the engineering community. Baker’s y-shaped structural system is the future of deg skyscrapers and may be the key to reaching unfathomable building heights. The buttressed core structural system has, without a doubt, revolutionized the structure and design of skyscrapers throughout the world.

THE FUTURE OF THE BUTTRESSED CORE STRUCTURAL SYSTEM SOM and Baker made history with the innovation of the buttressed core structural system, and the competition to build the tallest building in the world continues. The idea of a central core and three wings revolutionized the way that skyscrapers are structured and altered the approach that 5

Janna Brown Jenna Williams [12]"LexisNexis® Academic & Library Solutions." LexisNexis® Academic & Library Solutions. Emirates News Agency (WAM), 4 Apr. 2010. Web. [13] "Burj Dubai, the Shining Building." GUARDIAN Glass, 2010. Pdf. [14]"Code of Ethics." American Society of Civil Engineers. N.p., n.d. Web. 04 Mar. 2013 [15] Jones, Jenny. “World’s Tallest Building Must Be More Tall”. Civil Engineering (08857024)81.9(2011):16-17. Military & Government Collection.

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 Structural System for Burj Khalifa.” (Online Article). http://www.worldbuildingsdirectory.com/project.cfm?id=26 18 [3] Blum, Andrew. "Engineer Bill Baker Is the King of Superstable 150-Story Structures." Wired Magazine 27 Nov. 2007: n. pag. Web. [4] Abdelrazaq, Baker, Chung, Pawlikowski, Wang, and Yom. Integration of Design and Construction of the Tallest Building in Korea, Tower Palace III, Seoul, Korea. 10 Oct. 2004. South Korea, Seoul. [5] Baker, William, James Pawlikowski, and Bradley Young. "Reaching toward The Heavens."Civil Engineering Mar. 2010. [6] Baker, William. "Engineering an Idea: The Realization of the Burj Khalifa." Civil Engineering. [7] "Burj Khalifa Facts." Skyscrapercenter. Council on Tall Buildings and Urban Habita, n.d. Web. 07 Mar. 2013. [8] Bollinger, Peter. The Buttressed Core. Digital image. Wired Magazine. N.p., 27 Nov. 2007 [9] Baker, William, Stanton Korista, and Lawrence Novak. "Engineering the World's Tallest - Burj Dubai." Council on Tall Buildings and Urban Habitat (2008) [10] Burj Khalifa Typical Floor Plan. Digital image. Access Science. Silver Chair, 2010. Web. [11] Helms, Jeremy. "Header Menu." Industry Tap. N.p., 2011. Web.

ADDITIONAL REFERENCES Blum, Andrew. "Engineer Bill Baker Is the King of Superstable 150-Story Structures." Wired Magazine 27 Nov. 2007: n. pag. Web. "Design." Burj Khalifa RSS. InternetCont Ltd., n.d. Lowe, Aya, and Samia Badih. "Burj Khalifa Is an Outstanding Example of Sustainable Architecture." Latest News, Pictures, Video, Multimedia. N.p., 6 Jan. 2010. Web. "Skidmore, Owings & Merrill." International Directory of Company Histories. Ed. Tina Grant. Vol. 13. Detroit: St. James Press, 1996. 475-476. Gale Virtual Reference Library. Stephens, Suzanne. “Kingdom Come.” Architectural Record 200.5(2012):160 Business Source Complete. Web. 4 Mar. 2013

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

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