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, surpassedTaipei 101 by more than 60
percent . This massive jump in height can be attributed to the invention of the buttressed core
structural system. The essence of the system is a tripod-shaped structure in which a strong
central core anchors three building wings. 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 behaviour
and performed well in the wind tunnel, implying to engineers that it could be built much
higher. 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 .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 co dependence 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

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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 leap
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

support or reinforce the wall. Buttresses are fairly common on more ancient buildings, as a
means of providing support 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. 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. Placement
of smaller cores around stairs at ends of the wings can give an additional 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

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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’. It’s 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. 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 a 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.

Fig 2.1 The buttressed core system

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. Permitting a dramatic increase in height, its design employs conventional

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materials and construction techniques and was not precipitated by a change in materials or
construction technology. The essence of 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 the other 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 &
Merrill LLP (SOM). 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
over1,000 meters (exceeding the Burj Khalifa by more than 100meters).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 co-dependent and each is
supported (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
resistance and increase the moment of inertia, and as the building rises, each wing sets back
in a clockwise 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. 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 buttressed core allows for these skyscrapers
to go up tall and fast with enough usable floor space to maximize clients chances of making a
profit (Blum). The buttressed core’s design is most prevalent and well recognized in the
beautiful and extravagant building in Dubai the Burj Khalifa (WebBuildingsDirectory). The
Burj Khalifa offers a social impact as well bringing in extra profit and much publicity to
Dubai (Dowdey) Overall civil engineering 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 the future and show engineers that next step in innovation.

2.2 C ONSTRUCTING 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 as a whole but more as to

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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 accounted for the structure will fail, this is why they
found it so important to mention in 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. 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 admixtures 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 behaviours in certain support 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.

2.3 HEXAGONAL HUB

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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. 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 hammer headwalls
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 (support) the center core, with the center core in turn allowing the
wings to be supported 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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 Fig 2.3 Hexagonal hub

3. 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 supported by a 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 3). In
doing so, the “hub” became the primary lateral system of the building.

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 Fig 3.1 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 behaviour 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. Given 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

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main engineer behind the creation of the buttressed core structural system. 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. 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” .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:
“optimizing the tower structural system for strength and stiffness,” using gravity loads to
resist lateral loads, and limiting the torsion on the building. These goals were accomplished
through the y-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.

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 Fig 3.2 Tower palace iii

3.1 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 designing
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 .Despite the Tower Palace III’s solid structural behaviour,
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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 4. 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
designing 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”. Architects and engineers worked together to alter orthodox systems, resources, and
building methods to create the Burj Khalifa in Dubai.

Fig 4.1 Geometry of Burj Khalifa

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”. He also was responsible for meeting owner Emaar Properties Public Joint Stock
Company’s expectations. The Burj Khalifa needed to have enough width to support itself and
to be narrow enough to “create economically viable real estate for the client”. 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. 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 behaviour. In contrast to the case of Tower
Palace III, Burj Khalifa’s central core houses all vertical transportation with the exception of

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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. 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 $1.5billion 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 829.8meters to tip, 828 meters to
architectural top, and 584.5meters to highest occupied floor. It claimed these records by
beating out the Willis Tower (527 meters), Taipei 101 (508meters), and Shanghai World
Financial Center (474 meters),respectively. 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.

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 Fig 4.2 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.
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 counterclockwise 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

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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 8). In order to have an
efficient supertall 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 Khalifa’s base
columns are relatively thin and only slightly thicker than those at the top. 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 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

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 Fig 4.3 Typical floor plan

4.1 HEXAGONAL HUB

Clearly, the Burj Dubai has a much greater useable to unuseable space ratio than the
hypothetical Willis Tower. Although this building has much more useable space, it can only
reach a smaller maximum height. 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. 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.

4.2 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
support the core against lateral loads, and as the height of the building increases, one wing on
each tier sets back in a spiralling pattern, emphasizing the height of the tower. These setbacks
are also aesthetically pleasing for occupants of the tower because they maximize natural light

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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.
Setbacks usually require transfer beams to pass 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. These outrigger walls are only placed on the mechanical floors because they
would interfere with the usage of functional floors.

4.3 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 . 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” . Wind tunnel
models were used to “account for the cross wind effects of wind induced vortex shedding on
the building”. 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). 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 behaviour 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. 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
60MPa throughout the height of the building from bottom to top. There is a 232 m high steel

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structure in the upper part of the building, consisting of brace elements and with self
resistance against lateral and vertical loads, which is supported by the central core.

Fig 4.3.1 Disorganised vortex shedding

4.4 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 . 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 .
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

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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 torsionally 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 feeing the force of the winds, the other
two wings act as supports 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 a very
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 supported raft foundation.

Fig 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

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integral part of a skyscraper [7]. 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. Giving
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.

 4.6 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 account for about sixty percent of the world’s energy consumption. To minimize
unnecessary energy consumption, the Burj Khalifa utilizes a special building management
system with “smart lighting and mechanical control”. This system, created by Asea Brown
Boveri, Ltd., uses computer based systems to monitor and control electricity. 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 panels, each with an area of 2.7 square meters, lie on the roof of the office annexes.
These panels 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. 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. 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. The water collected
is used for irrigation of the landscape around the Burj Khalifa and is enough to fill 14
Olympic sized swimming pools. 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

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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. 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.

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 5. LAS VEGAS TOWER
The design of Las Vegas Tower (Crown Las Vegas) marked the next step in the evolution of
the buttressed core. In early 2006, SOM began working on the design of a 575 m hotel tower
located on the Las Vegas Strip (see figure 9). As in its predecessors, each of the tower’s three
wings buttresses the other via a central core. However, rather than stepped setbacks, Las
Vegas Tower has a shape that changes in elevation, causing the tower’s width to continually
vary. In this way, wind vortices never get organized. Furthermore, continual changes in the
tower’s footprint required the loads to be moved to other elements besides the wing walls.
This was accomplished by locating the stair at the end of the corridor(fig 5.2).

Fig 5.1 Las Vegas Tower

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The concrete around the stair, somewhat like the chord of a truss, acts as a major structural
element but moves toward the center of the building as it tapers at the top. In this way it does
not require the stair transfers that were necessary in the Burj Khalifa and permits a much
smoother load transfer than a solution that relies on setbacks. The stair core also provides for
a large amount of structure placed near the end of each wing, thereby significantly increasing
the tower’s moment of inertia. However, the system is similar to the Burj Khalifa in that it
employs direct outriggers connecting the perimeter columns to the interior core walls at each
mechanical floor.

Fig 5.2 Stair at the centre of corridor

(The project was ultimately never built.) In 2009 SOM embarked on a design competition for
what is set to be the next world’s tallest building, Kingdom Tower, in Jeddah, Saudi Arabia.
At more than 1,000 m, the mixed use tower’s elongated, triangular shape is a direct
descendant of the Tower Palace III, Burj Khalifa, and Las Vegas Tower paradigm and is
derived from an optimized structural form for strength and wind performance. Referred to as
the stayed buttressed core, this structural system was developed from an extensive analysis of
the construction history of its predecessors and of the methods that were employed. SOM
proposed two schemes for this tower, one with columns and one without. The scheme with
columns is similar to that used for the Burj Khalifa: columns of the perimeter blade type
located in line with interior transverse core wall elements. Like their predecessors, these

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columns required linkage to the core via direct outriggers, although distributed link beams
also were considered. Early in the design process, it was realized that there was an
opportunity to create the next generation of the buttressed core and to eliminate these
columns and, with them, the outriggers, thereby facilitating construction and increasing
efficiency. A rigorous study was conducted to determine the optimum wall geometry with
respect to system efficiency and stiffness.

Fig 5.3 Wall geometry analysis

Thus, the column-free scheme was born. Like the system considered in 2003 for the Burj
Khalifa, this system used a central structural core with short transverse walls that continue
into each of the three wings, supporting cantilevered floors and a column-free perimeter.
Stairs with surrounding walls are located at the end of each wing and scale back a nominal
amount at each level to establish the building’s taper, thereby eliminating any setbacks.

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 Fig 5.4 Cantilever floors
The tripartite floor geometry, in combination with a shallow lease span, produces a
breathtaking structure of unencumbered space that in its height and panoramic views realizes
the full potential of the buttressed core concept. Because of the amazing stiffness of this
refined structural system, SOM engineers were able to scale the system to achieve a much
taller building using virtually the same concrete quantities quantities per square meter as in
the Burj Khalifa, which is already very efficient. This new structural system also eliminates
the need for outriggers and perimeter columns and is easily constructed within a standardized
formwork system, thus greatly simplifying and accelerating construction. Tapering as they
rise, the symmetrical internal core elements are sized to maximize their footprint and allow
the building to move loads efficiently to the ground while shortening the construction
schedule through the elimination of perimeter columns, complex outrigger trusses, and
similar transfer elements.

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 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”. 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”. 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. 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”. 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.

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 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 designing 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. 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 . 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” .
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”. 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 (surpassing 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 designing
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 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.,

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