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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 thecompletion of 1 World Trade Center (1972) and Taipei 101

(2004), there was only a 22 percent increase in the height of theworlds tallest building. In 2010, the Burj Khalifa claimed thetitle at 828 m, eclipsing Taipei 101 by more than 60 percent.With its innovative buttressed core, the tower represents a majorleap in structural design, elicited by a change in the approach to

the tall building problem through an examination of scale.

By William F. Baker, P.E., S.E., F.ASCE, and

James J. Pawlikowski, S.E., LEED AP, M.ASCE

Higher and :

The EvolutiButtressed


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THROUGHOUT THE HISTORY of tallbuildings, structural engineers have in-vented the means to go higher. In the1970s Fazlur R. Khans tube conceptwas a dramatic shift from the tra-ditional portal frame system usedon such structures as the Empire

State Building. Later develop-ments, including the core plus outrigger system,also provided architects with the tools to designtaller, more efcient buildings. However, the re-sulting growth was gradual, each innovationmarking a point on the progressive scale of thetall building.

The buttressed core is a different species.Permitting a dramatic increase in height, itsdesign employs conventional materi-als and construction techniques andwas not precipitated by a changein materials or construc-tion technology. The es-sence of the systemis a tripod-shapedstructure in whicha strong centralcore anchors threebuilding wings. Itis an inherently stablesystem in that each wing isbuttressed by the other two.The central core provides thetorsional resistance for the

building, while the wings pro-vide the shear resistance andincreased moment of inertia(see gure 1, typical oor planof the Burj Khalifa). The but-tressed core represents a con-ceptual change in structuraldesign whose evolutionary de-velopment began with TowerPalace III, designed by Chica-go-based Skidmore, Owings& Merrill LLP (SOM).

Completed in 2004, Tow-

er Palace III, located in Seoul,South Korea, promoted a newstandard in high-rise residen-tial development (see figure2). Its tripartite arrangementprovides 120 degrees betweenwings, affording maximumviews and privacy. AlthoughChicagos Lake Point Towerset the architectural precedent

for the residential high-rise, the design of Tower Palace IIIrevealed a new structural solution for the supertall residen-

tial tower.Tower Palace III was originally designed at morethan 90 stories, its height supported by a Y-shapedoor plan. Because its architectural design called forelevators within the oval oor plate of each wing,

SOM engineers opted to connect the elevators viaa central cluster of cores (parts a and b of gure 3).In doing so, the hub became the primary lateral

system of the building. At the two upper me-chanical oors, the perimeter columns also wereengaged to assist in resisting lateral loads bymeans of virtual outriggers (oor plates aboveand below in conjunction with a perimeter beltwall). While not as effective as direct connec-

tions, these virtual outriggers spared thebuilders the numerous connection

and construction problems typi-cally associated with direct

outriggers (see gure 4).Throughout the

design process, thebuilding exhibit-ed very good struc-tural behavior andperformed well in

the wind tunnel, and itbecame obvious to the engi-neering team that the struc-ture could go much higher.However, because of zoning is-

sues, the design of the towerstallest wing was cut from 93to 73 stories (the other wingswere then elevated to compen-sate for the loss of area). De-spite the decrease in height,the project provided the SOMteam with the opportunity toexplore a new approach to thetall building problem. GivenTower Palace IIIs efficiency,the structural design team in-ferred that, if a project had a

sufciently large parcel, thissystem could be used in build-ing at extreme heights.

In early 2003, soon af-ter completing the design ofTower Palace III, SOM wascontacted about a potentialsupertall building in Dubayy(Dubai), part of the Unit-ed Arab Emirates. (See The S O

M

| N I C K

M E R R I C K

H E D R I C H

B L E S S I N G

, O P P O S I T E ;

S O M

, T O P ;

S O M

| H

. G .

E S C H

, B O T T O M

0885-7024/12-0010-0058/$30 .00 PER ARTICLE O C T O B E R 2 0 1 2 C i v i l E n g i n e e r i n g

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Burj Khalifa Triumphs by WilliamF. Baker, P.E., S.E., F.ASCE , CivilEngineering, March 2010, pages4455.) On March 1 of thatyear, the team went to NewYork to be interviewed forthe project, and it was

agreed there that a briefidea competitionwould be held involv-ing SOM and various otherinvited teams. Given thesuccess of Tower PalaceIII and its potential to be de-veloped to even greater heights, theSOM team elected to use this structuralsystem for what would later become the BurjKhalifa (see gure 5).

Throughout the design process, SOM engineersmade critical changes to the Tower Palace III design

that were essential to the evolution of the Burj Khalifasbuttressed core. The design of the towers central corerelied upon close collaboration on the part of SOM ar-chitects and engineers, and that multidisciplinary ap-proach successfully t all of the towers elevators andoperating systems within the core while maintaininggood structural behavior. In contrast to the case of Tow-er Palace III, Burj Khalifas central core houses all ver-tical transportation with the exception of egress stairswithin each of the wings (see gure 6).

Each of the three wings forming the Burj Khalifas

buttressed core is on a 9 m module.As in Tower Palace III, the walls ineach wing of the Burj Khalifa were

initially spread apart in sucha way as to separate the living

components from the bath andkitchen components. This provided

four interlocking tubes, but the dimen-sions were much greater. This plan later

proved problematic because there were nu-merous doors in the structure and little exibil-ity in unit layout. It was thus difcult to comply

with Dubayy code requirements, which dictate accessi-bility to natural light in the kitchen. As a result, the team

embarked on a series of studies to see if the central corecould resist all of the torsional effects of the building. Follow-ing a round of parametric studies carried out in the autumn of2003, it was clear that the central core had enough strengthand stiffness to serve as the buildings torsional hub. Also in2003, the wing walls were adjusted so that the primary wallsnow lined the corridors at the center of each wing, instead ofprotruding into the units. Besides improving the efciencyof the units, this adjustment improved the efciency of theentire structure.

Studies were also carried out to assess the possibility ofeliminating the perimeter columns by using cantilever beamsfrom the core walls. After SOM was selected to design the Burj

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Khalifa, the engineering team immediately tested the towersinitial geometry in the wind tunnel, only to discover that ithad large movements and base moments.

Upon further analysis, it was discovered that the re-sults were more closely related to the geometryand orientation of the tower than to the struc-tural system. Therefore, the dynamic proper-ties of the structure were manipulated in or-der to minimize the harmonics with thewind forces. Engineers were able to ac-complish this by essentially tuningthe building as if it were a musicalinstrument in order to avoid theaerodynamic harmonics that areresidual in the wind.

A key component of the

Burj Khalifas structuraldesign was managinggravity. This meantmoving the gravityloads to where theywould be most use-ful in resisting the lat-eral loads. Structural en-gineers manipulated thetowers setbacks in such away that the nose of the tier

above sat on the cross-walls of the tier below, yielding greatbenets 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 moduleand a philosophy of no transfers (gure 7).Several rounds of high-frequency force bal-

ance tests were undertaken in the wind tun-nel as the geometry of the tower evolved

and as the tower was rened architectur-ally, the setbacks in the three wings fol-

lowing a clockwise pattern (in contrastto the counterclockwise pattern in

the original scheme). After eachround of wind tunnel testing,

the data were analyzed andthe building was reshaped

to minimize the wind ef-fects and accommodate

unrelated changes inthe clients program.In general, the num-

ber and spacing of thesetbacks changed, as did

the shape of the wings.The designers also noticed

that the force spectra for cer-tain wind directions showed

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FIGURE 4

FIGURE 6


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less excitation in the important frequency rangewhen winds impacted the pointed, or nose, endof a wing than when they impacted the tails be-tween the wings.

This was kept in mind when selecting the ori-entation of the tower relative to the most frequentdirections of strong wind in Dubayy, which arefrom the northwest, south, and east. The carefulselection of the towers orientation, along with

its variant setbacks, resulted in substantial reduc-tion of wind forces. By confusing the wind, thedesign encourages disorganized vortex sheddingover the height of the tower (see gure 8).

In order to have an efcient supertall building,it is best to use all the vertical elements for bothgravity and wind loads. In order to achieve thison the Burj Khalifa, it was necessary to engageall of the perimeter columns of the structure. Be-cause of the towers extreme height, the virtualoutrigger used on Tower Palace III was replacedby a direct outrigger. In addition to engaging theperimeter for lateral load resistance, the outrig-gers allow the columns and walls to redistributeloads several times throughout the buildingsheight. This helps control any differential short-ening between the columns and the core. By thetime the building meets the ground, the loads inthe walls are somewhat ordinary, and in contrastto the case of many buildings in which the col-umns at the base are massive, most of the BurjKhalifas base columns are relatively thin andonly slightly thicker than those at the top.

The Burj Khalifas structural system was cre-ated with a conscious effort to conform to and

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The Burj Kstructural sy

created with effort to concomplemen

construction

FIGURE 7

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complement current construction technology. The goal wasto use a highly organized system with conventional elementsthat would provide a high repetition of formwork. Initiallythe team contemplated a composite oor framing system, aswell as an all-concrete oor framing scheme. It was later de-cided that the all-concrete scheme was more appropriate andeconomical. Although the towers oor plate changes as thestructure ascends, the segments near the core repeat them-selves for as much as 160 levels. As the loads accumulate fromthe top down, the sizes of the structural elements are relative-ly constant since walls were added as the loads accumulated

The design of Las Vegas Tower (Crown Las Vegas) markedthe next step in the evolution of the buttressed core. In early2006, SOM began working on the design of a 575 m hotel

tower located on the Las Vegas Strip(see gure 9). As in its predecessors,each of the towers three wings but-tresses the other via a central core.

However, rather than stepped set-backs, Las Vegas Tower has a shape thatchanges in elevation, causing the tow-ers width to continually vary. In thisway, wind vortices never get organized.Furthermore, continual changes in thetowers footprint required the loads tobe moved to other elements besides the

wing walls. This was accomplished bylocating the stair at the end of the corri-dor (see gure 10). The concrete aroundthe stair, somewhat like the chord ofa truss, acts as a major structural ele-ment but moves toward the center ofthe building as it tapers at the top. Inthis way it does not require the stairtransfers that were necessary in the BurjKhalifa and permits a much smootherload transfer than a solution that relies


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However, rather than steppedsetbacks, Las Vegas Tower

has a shape that changes inelevation, causing the towerswidth to continually vary.

FIGURE 9

FIGURE 10


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on setbacks. The stair core also provides for a large amount ofstructure placed near the end of each wing, thereby signi-cantly increasing the towers moment of inertia. However,the system is similar to the Burj Khalifa in that it employsdirect outriggers connecting the perimeter columns to theinterior core walls at each mechanical oor. (The project wasultimately never built.)

In 2009 SOM embarked on a design competition for whatis set to be the next worlds tallest building, Kingdom Tower,in Jeddah, Saudi Arabia. At more than 1,000 m, the mixed-use towers elongated, triangular shape is a direct descendant

of the Tower Palace III, Burj Khalifa, and Las Vegas Towerparadigm and is derived from an optimized structural formfor strength and wind performance (see gure 11). Referredto as the stayed buttressed core, this structural system was de-veloped from an extensive analysis of the construction historyof its predecessors and of the methods that were employed.

SOM proposed two schemes for this tower, one with col-umns and one without. The scheme with columns is similarto that used for the Burj Khalifa: columns of the perimeterblade type located in line with interior transverse core wallelements. Like their predecessors, these columns required

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FIGURE 11


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linkage to the core via direct outriggers, although distributedlink beams also were considered. Early in the design process,it was realized that there was an opportunity to create thenext generation of the buttressed core and to elimi-nate these columns and, with them, the outriggers,thereby facilitating construction and increasing ef-ciency. A rigorous study was conducted to deter-mine the optimum wall geometry with respect tosystem efciency and stiffness (see gure 12). Thus,the column-free scheme was born. Like the systemconsidered in 2003 for the Burj Khalifa, this system

used a central structural core with short transversewalls that continue into each of the three wings,supporting cantilevered oors and a column-freeperimeter (gure 13). Stairs with surroundingwalls are located at the end of each wing andscale back a nominal amount at each levelto establish the buildings taper, there-by eliminating any setbacks. Thetripartite floor geometry, incombination with a shal-low lease span, pro-duces a breath-taking structure

of unencumberedspace that in itsheight and pan-oramic views realizesthe full potential of the buttressed coreconcept.

Because of the amazing stiffness ofthis rened structural system, SOM en-gineers were able to scale the systemto achieve a much taller building us-ing virtually the same concrete quanti-

ties per square meter as in the Burj Khalifa, which is alreadyvery efcient. This new structural system also eliminates the

need for outriggers and perimeter columns and is easilyconstructed within a standardized formwork system,thus greatly simplifying and accelerating construc-tion. Tapering as they rise, the symmetrical internalcore elements are sized to maximize their footprintand allow the building to move loads efcientlyto the ground while shortening the constructionschedule through the elimination of perimeter col-umns, complex outrigger trusses, and similar trans-

fer elements.The evolution of the buttressed core traces thedevelopment of a simple yet powerful structur-

al idea. This idea was developed into an appro-priate and successful system for each of the

buildings described here. With each build-ing, this system was further rened, re-

ecting both its exibility and itspotential. The buttressed core

has evolved into a systemthat truly incorpo-

rates the ideals ofstructural efcien-

cy, constructabil-ity, and architec-

tural function andmakes it possible to

produce buildings of great height. CE

William F. Baker, P.E., S.E., F.,ASCE , is a partner of Skidmore, Owings & Merrill LLP in Chicago, and James J. Pawlikow- ski, S.E., LEED AP, M.ASCE , is an associ- ate director of the rm.


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, B A K E R S P H O T O ;

S O M

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A L L R E M A I N I N G

A rigorous study was conducted to derespect to system efciency and stiffn

Baker Pawlikowski

FIGURE 12

FIGURE 13

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