The Buttressed Core Structural System 1a403a

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 indepth understanding of the buttressed core structural system.

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1-Introduction .Throughout the history of tall buildings, structural engineers have invented the means to go higher. In the 1970s Fazlur R. Khan’s tube concept was a dramatic shift from the traditional portal frame system used on such structures as the Empire State Building. Later developments, including the core plus outrigger system, also provided architects with the tools to design taller, more efficient buildings. However, the resulting growth was gradual, each innovation marking a point on the progressive scale of the tall building. The buttressed core is a different species. Permitting a dramatic increase in height, its design employs conventional materials and construction techniques and was not precipitated by a change in materials or construction technology. The development of the buttressed core structural system led to a paradigm shift in tall building design that brought a dramatic increase in the height of buildings. In the 32 years between the completion of 1 world Trade center 1972 and Taipei 101 2004, there was only a 22 percent increase in the height of the world’s tallest building. In 2010, the Burj khalifa claimed the title at 828 m, eclipsing Taipei 101 by more than 60 "percent. With its innovative buttressed core, the tower represents a major lead in structural design, elicited by a change in the approach to the tall building problem through an examination of scale. A buttress is an architectural structure built against or projecting from a wall which serves to or reinforce the wall. Buttresses are fairly common on more ancient buildings, as a means of providing to act against the lateral (sideways) forces arising out of the roof structures that lack adequate bracing. The essence of the system is a tripod-shaped structure in which a strong central core anchors three building wings.

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Buttressed core system is a solution to spread gravity loads out from the center and also use that to give improved lateral stabilization with the ability to construct higher buildings. The design of a building with buttressed core is a structure where the core is stabilized with outgoing wings. The central core, providing torsional resistance, is attached with building wings, providing shear resistance and prohibiting overturning moment by an increased moment of inertia. The wing’s wall could be formed as an elongated box instead of one continuous piece given better torsional resistance. A virtual or direct outrigger can be used to engage the perimeter columns, stabilizing each wing. If smaller shear walls are placed orthogonally and connected to the wings the need for columns can be abandoned. This paper will describe and demonstrate the use of the buttressed core, the newest and most cutting edge design being used in the infrastructures of some of the tallest and the tallest building in the world, these otherwise known as skyscrapers. The infrastructures and designs of these buildings will be explained thoroughly as well as the direction that these skyscrapers and modern buildings are heading for. Further discussion will show the values of the known and proven advantages of this innovation of the buttressed core. Its three Wing design which extend out of the central core and firmly anchor the skyscrapers will be described and told as well its use in the future building of our cities most iconic landmarks. In this paper, the buttressed core will be examined in detail, describing the different components and parts which make up the buttressed core and the materials which go into making it like the use of fly ash in the cement of the core (sheath), describing in part how it operates as a whole, making the world’s tallest skyscrapers more structurally sound even at their ridiculous heights.

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2- 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 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, sixsided 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 giveand-take between the core of the building and its wings are the key to the structural system and allow for taller, more stable skyscrapers.

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3. The buttressed core system A buttressed core framing system is built around a strong central core, which is further reinforced by shear walls or other rigid elements that radiate out from it, in manner similar to a tripod though not necessarily three in number. Buttressed core, is a kind of three-winged spear that allows stability, viably usable space (as in not buried deeply and darkly inside a massively wide building) and limits loss of space for structural elements.

Figure (3-1) The Buttressed core System

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The Tower palace III introduced the engineering and architectural worlds to anentirely new approach to building skyscrapers, known as the buttressed core structuralsystem. This structural system then evolved and extended its potential for incredible building heights in the design and construction of the building that currently boaststhe title of tallest in the world, the Burj khalifa. Permitting a dramatic increase in height, its design employs conventional materials and construction techniques and was not precipitated by a change in materials or construction technology. The essenceof the system is a tripod-shaped structure in which a strong central core anchors three building wings. It is an inherently stable system in that each wing is buttressed by theother two. The central core provides the torsional resistance for the building, while the wings provide the shear resistance and increased moment of inertia. The buttressed core represents a conceptual change in structural design whose evolutionary development began with Tower palace III, designed by Chicago-based Skidmore,Owings and Merrill (SOM). The designers of the Burj Khalifa, asustainable building, utilized a number of techniques to reduce the building’s energyconsumption. The much-antiated Kingdom Tower will also utilize the buttressedcore structural system to climb to a height of over 1,000 meters (exceeding the Burj Khalifa by more than 100meters).The crue 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. The torsional resistance for the building is supplied by the strong, six-sided central core or (hexagonal hub). The three wings afford the shear resistanceand increase the moment of inertia, and as the building rises, each wing sets back in aclockwise pattern. 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.

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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.The buttressedcore allows for these skyscrapers to go up tall and fast with enough usable floor spaceto maximize clients chances of making a profit[3]. The buttressed core’s design is most prevalent and well recognized in the beautiful and extravagant building in Aubaithe Burj Khalifa (Web Buildings Directory). The Burj Khalifa offers a social impactas well bringing in extra profit and much publicity to Dubai Overall civilengineering in the future is set to explode and people and cities want more beautiful and taller buildings, the buttressed core allows for us to create these buildings of thefuture and show engineers that next step in innovation.

3.1 Constructing the core The buttressed core may seem to many to have a more ‘simple’ design to its structure. To engineers, the buttressed core is a thing of incredible ingenuity and there is much more to what meets the eye when it comes to the recognizable, “Y” design. As engineers, to us it is not what a piece of machinery or what a structure may look like a whole but more as to how the whole is made up of from the many different parts. The buttressed core is something which is an incredible innovation as a whole but is also something that is made up of many parts and without each and every part ed for the structure will fail, this is why they found it so important to mention their conference paper the actual construction of a buttressed core and what goes into giving it such amazing structural quality. One of the major components when it comes to constructing the buttressed core is the actual cement used to make it. One of the interesting things about this structural innovation is that it actually is not pure cement or concrete.

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What is used to make the buttressed core is a newer and more cost effective way to make cement-like substances known as fly ash. The development and use of mineral ixtures like fly ash are becoming more common in the construction industry mainly due to the consideration of a more cost-effective, energy saving, and the environmental production and conservation of resources. There is even currently a study that is looking at replacing cement in concrete more and more with the more flexural fly ash, testing its behaviors in certain beams and other structural uses. In the buttressed core the fly ash is a key component and is even growing more popular in the entire world of construction. Another key component in the construction of the buttressed core is its intriguing design. The buttressed core is designed in such a way that it makes it perfect for constructing such amazingly tall heights. One of the major issues when it comes to constructing buildings that challenge the heights of the tallest in the world is the wind. At very tall altitudes the wind can be so strong at times that it causes the structure itself to sway and this can be very dangerous if engineers do not use the correct type of structures when building these huge buildings. For instance the Burj Khalifa that is known to all as the tallest building in the world, as stated earlier uses the buttressed core for its amazing structural strength. This tri-axial design consists of three tiers that are staggered throughout construction of each floor as the building gets taller and taller. This design is the key to the building itself staying in that safe zone where the building can sway with the wind but not to the point where it becomes a dangerous risk. This three-tier design allows the wind to not hit one side directly or head on, diverting the wind from the hitting the building straight on at any point.

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3.2 Hexagonal hub 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 torsional 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).

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Figure (3-2) Hexagonal hub

4. Evolution of buttressed core system Completed in 2004, Tower palace III, located in Seoul, South Korea, promoted a new standard in high-rise residential development. Its tripartite arrangement provides 120 degrees between wings, affording maximum views and privacy. Although Chicago’s lake point Tower set the architectural precedent for the residential high-rise, the design of Tower palace III revealed a new structural solution for the super tall residential tower. Tower palace III was originally designed at more than 90 stories, its height ed by a By Y-shaped floor Plan. Because its architectural design called for elevators within the oval floor Plate of each Wing, (SOM) engineers opted to connect the elevators via a central cluster of cores (figure 4-1). In doing so, the hub became the primary lateral system of the building

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Figure (4-1) Connection of elevators via central cores At the two upper mechanical floors, the perimeter columns also were engaged to assist in resisting lateral loads by means of virtual outriggers (floor Plates above and below in conjunction with a perimeter belt wall). While not as effective as direct connections, these virtual outriggers spared the builders the numerous connection and construction problems typically associated with direct outriggers. Throughout the design process, the building exhibited very good structural behavior and performed well in the wind tunnel, and it became obvious to the engineering team that the structure could go much higher. However, because of zoning issues, the design of the tower’s tallest wing was cut from 93 to 73 stories (the other wings were then elevated to compensate for the loss of area). Despite the decrease in height, the project provided the SOM team with the opportunity to explore a new approach to the tall building problem.

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Civen Tower palace III’s efficiency, the structural design team inferred that, if a project had a sufficiently large parcel, this system could be used in building at extreme heights. 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, allresidential building in the Kingman 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: “optimize [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 shaped structural system, which was designed to maximize views from the tower and for the intake of natural light. Engineers and architects then discovered that this shape was incredibly stable and strong [4].

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Figure (4-2) the Tower Palace III

4.1 Limitations of the Tower Palace III 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.

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5. 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 “resisting 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.

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Figure (5-1) Burj Khalifa

Throughout the design Process, SOM engineers made critical changes to the Tower palace III design that were essential to the evolution of the Burj khalifa’s buttressed core. The design of the tower’s central core relied upon close collaboration on the part of SOM architects and engineers, and that multidisciplinary approach successfully fit all of the tower’s elevators and operating systems within the core while maintaining good structural behavior. In contrast to the case of Tower Palace III, Burj Khalifa’s central core houses all vertical transportation with the exception of egress stairs within each of the wings. Each of the three wings forming the Burj Khalifa’s buttressed core is on a 9 m module.

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As in Tower palace III, the walls in each wing of the Burj Khalifa were initially spread apart in such a way as to separate the living components from the bath and kitchen components. This provided four interlocking tubes, but the dimensions were much greater. This plan later proved problematic because there were numerous doors in the structure and little flexibility in unit layout. It was thus difficult to comply with Dubayy code requirements, which dictate accessibility to natural light in the Kitchen. As a result, the team embarked on a series of studies to see if the central core could resist all of the torsional effects of the building. Following a round of parametric studies carried out in the autumn of 2003, it was clear that the central core had enough strength and stiffness to serve as the building’s torsional hub. Also in 2003, the wing walls were adjusted so that the primary walls now lined the corridors at the center of each wing, instead of protruding into the units. Besides improving the efficiency of the units, this adjustment improved the efficiency of the entire 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 wall construction” with a hexagonal hub and three buttressed wings [5].

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Fig (5-2) Vertical transportation in central core Upon further analysis, it was discovered that the results were more closely related to the geometry and orientation of the tower than to the structural system. Therefore, the dynamic properties of the structure were manipulated in order to minimize the harmonics with the wind forces. Engineers were able to accomplish this by essentially ‘tuning’ the building as if it were a musical instrument in order to avoid the aerodynamic harmonics that are residual in the wind. A key component of the Burj Khalifa’s structural design was ‘managing gravity’. This meant moving the gravity loads to where they would be most useful in resisting the lateral loads. Structural engineers manipulated the tower’s setbacks in such a way that the nose of the tier above sat on the cross-walls of the tier below, yielding great benefits for both tower strength and economy. Engineers also employed a series of ‘rules’ to simplify load paths and construction. These included a rigorous 9 m module and a philosophy of no transfers.

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several rounds of high-frequency force balance tests were undertaken in the wind tunnel as the geometry of the tower evolved and as the tower was refined architecturally, the setbacks in the three wings following a clockwise pattern (in contrast to the counter clockwise pattern in the original scheme). After each round of wind tunnel testing, the data were analyzed and the building was reshaped to minimize the wind effects and accommodate unrelated changes in the client’s program. In general, the number and spacing of the setbacks changed, as did the shape of the wings. The designers also noticed that the force spectra for certain wind directions showed less excitation in the important frequency range when winds impacted the pointed, or nose, end of a wing than when they impacted the tails between the wings. This was kept in mind when selecting the orientation of the tower relative to the most frequent directions of strong wind in Dubayy, which are from the northwest, south, and east. The careful selection of the tower’s orientation, along with its variant setbacks, resulted in substantial reduction of wind forces. By ‘confusing ‘the Wind, the design encourages disorganized vortex shedding over the height of the tower (see figure 5-3). In order to have an efficient super tall building, it is best to use all the vertical elements for both gravity and wind loads. In order to achieve this on the Burj Khalifa, it was necessary to engage all of the perimeter columns of the structure. Because of the tower’s extreme height, the virtual outrigger used on Tower palace III was replaced by a direct outrigger. In addition to engaging the perimeter for lateral load resistance, the outriggers allow the columns and walls to redistribute loads several times throughout the building’s height. This helps control any differential shortening between the columns and the core. By the time the building meets the ground, the loads in the walls are somewhat ordinary, and in contrast to the case of many buildings in which the columns at the base are massive, most of the Burj Khalif’s base columns are relatively thin and only slightly thicker than those at the to". The Burj Khalifa’s structural system was created with a conscious effort to conform to and complement current construction technology. The goal was to use a highly organized system with conventional elements that would provide a high repetition of formwork. Initially the team

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contemplated a composite floor framing system, as well as an all-concrete floor framing scheme. It was later decided that the all-concrete scheme was more appropriate and economical. Although the tower’s floor plate changes as the structure ascends, the segments near the core repeat themselves for as much as 160 levels. As the loads accumulate from the top down, the sizes of the structural elements are relatively constant since walls were added as the loads accumulated.

Figure (5-3) typical floor plan

5.1 Hexagonal hub Clearly, the Burj Dubai has a much greater useable to un useable space ratio than the hypothetical willis Tower. Although this building has much more useable space, itcan only reach a smaller maximum height. The design that SOM created alsominimizes 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 occursmore quickly in thinner walls and columns, the perimeter column

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thickness mimicsthe typical corridor wall thickness. The thickness of these perimeter columns is determined by stress on the interior corridor walls. 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.

5-2 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]. The wings were constructed such that the perimeter columns on each floor lined up with the walls below them, providing a smooth load path, 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.

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5.3 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 aero elastic 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. High strength concrete was used in the Burj Khalifa, varying in strength between 80 MPa and 60 MPa throughout the height of the building from bottom to top. There is a 232 m high steel structure in the upper part of the building, consisting of brace elements and with selfresistance against lateral and vertical loads, which is ed by the central core.

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Figure (5-4) disorganized vortex shedding

5.4 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 deeprooted foundations [5].

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What this means is that at such extreme heights the things experience on the surface have to be magnified. The Burj Khalifa is immense in size and what this means is that gravity affects it greatly pushing down on all parts of the incredible building, meaning that to withstand such a large force of gravity the strength of the building in the upward direction must be incredibly powerful. The upward force must be large enough to withstand gravity so that the structure itself does not collapse in on itself. The buttressed core allows for structures to become very rigid and give them the strength vertically to resist the forces that cause it to collapse. This is directly accredited to the design of the buttressed core where the three wings are attached to a very strong central core. The central core is the key factor in giving the structure the strength to withstand intense weight of gravity. To be strong vertically as well as torsion ally or otherwise the ability to resist twisting as a result of winds. The Burj Khalifa was constructed in Dubai where the average wind speed over fifty years has been just over twenty-two miles per hour. For the Burj Khalifa to stand at a height where no other structure has ever been built it would need to have a design where the resistance of the natural act to twist in the high winds is fought against. The buttressed core is what allows the structure to stand at the height of over 800 meters in the air. The three wings use each other to build that strength. If one wing is feeling the force of the winds, the other two wings act as s to help keep it from twisting. This design is perfect to have the Burj Khalifa stand at such a mind-boggling height without twisting on itself. All these components of the buttressed core gives structures like the Burj Khalifa Avery efficient structure for the fact that the gravity load resistant system is utilized so it can maximize its use in resisting the lateral forces like that of the incredible wind gusts. On the top of the Burj Khalifa there is about a 230 meter tall spire and the complete structure of the tower founded on a 3.7-meter thick reinforced concrete pile ed raft foundation.

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Figure (5-5) picture of foundation

Constructing the buttressed core involves very precise and exact measurements, like every other part of a well bit structure it takes much time and effort to be able to construct such an integral part of a skyscraper. Every step in the process of constructing the buttressed core is a key to its success and holds all the answers to how it allows such amazing structural power for these ‘super’ skyscrapers that are reaching new heights every day. Civing structures the amazing ability to both resist the vertical force of gravity as well as having the lateral strength to resist the force of the wind.

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5.5 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 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 Asia 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].

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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.

6. 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.

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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

7. 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 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

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the world’s tallest building in 2018 [15]. In 2009, Prince AL weed 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.

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8. 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 evolution of the buttressed core traces the development of a simple yet powerful structural idea. This idea was developed into an appropriate and successful system for each of the buildings described here. With each building, this system was further refined, reflecting both its flexibility and its potential. The buttressed core has evolved into a system that truly incorporates the ideals of structural efficiency, constructability, and architectural function and makes it possible to produce buildings of great height.

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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 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 Super stable 150-Story Structures." Wired Magazine 27 Nov. 2007: n. page. 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." Skyscraper center. Council on Tall Buildings and Urban Habitat, 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. [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.

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