Structural design of Burj Khalifa

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Design and construction of the world’s tallest building: The Burj DubaiPUSHING TECHNOLOGY TO NEW HEIGHTS

By William F. Baker, P.E., S.E., James J. Pawlikowski, S.E., LEED AP

Burj Dubai, designed by and copyright to Skidmore, Owings and Merrill LLP



The Burj Dubai Tower will open in January of 2010. The final height of the more-than-160 story tower is yet to be revealed.

The Burj Dubai Tower is the world’s tallest structure, passing all previous height records. Such a project by

necessity requires pushing current analysis, material, construction technologies, and building systems to literally

new heights. However, as such a building height has never before been attempted, it is also necessary to ensure

all technologies and methods used are of sound development and practice. As such, the designers sought to beable to use conventional systems, materials, and construction methods — modified and utilized in new capacities

— to achieve such a lofty goal.

The 160-plus-story Burj Dubai Tower is the centerpiece of a $20 billion multi-tower development located justoutside of downtown Dubai. The Burj Dubai project consists of the tower itself, as well as an adjacent podium

structure, and separate six-story office annex and two-story pool annex. The 280,000-square-meter (m2) (or 3

million-square-foot, ft2) reinforced concrete multi-use tower is predominantly residential and office space, but italso contains retail space and a Giorgio Armani hotel. The tower and podium structures — combined 465,000

m2 (5 million ft2) — are currently under construction, and the project is scheduled for completion in late 2009.

Architectural design The primary design concept of the tower is an organic form with tri-axial geometry and spiraling growth that can

be easily seen in the final design. Additionally, traditional Islamic forms were utilized to enrich the tower’s design,and to incorporate visual references to the culture and history of the surrounding region. As such, the floor plan

of the tower consists of a tri-axial, “Y” shaped plan, formed by having three separate wings connected to acentral core. As the tower rises, one wing at each tier sets back in a spiraling pattern, further emphasizing itsheight. The Y-shape plan is ideal for residential and hotel use in that it allows the maximum views outward

without overlooking a neighboring unit. The wings contain the residential units and hotel guest rooms, with thecentral core housing all of the elevators and mechanical closets. The tower is serviced by five separate

mechanical zones, located approximately 30 floors apart over the height of the building. Located above theoccupied reinforced concrete portion of the building is the structural steel spire, housing communication and

mechanical floors, and completing the architectural form of the tower. The result is an efficient building in termsof its functionality, structural system, and response to wind, while still maintaining the integrity of the initial design

concept.



The tower’s Y-shaped floor plan not only has aesthetic and functional advantages, but isalso ideal for providing a high performance, efficient structure.

Structural system description

The tower’s Y-shaped floor plan not only has aesthetic and functional advantages, but also is ideal for providinga high-performance, efficient structure. The structural system for the Burj Dubai can be described as a

“buttressed-core” and consists of high-performance concrete wall construction. Each of the wings buttresses theothers via a six-sided central core, or hexagonal hub. This central core provides the torsional resistance of the

structure, similar to a closed pipe or axle. Corridor walls extend from the central core to near the end of eachwing, terminating in thickened hammer head walls. These corridor walls and hammerhead walls behave similar to

the webs and flanges of a beam to resist the wind shears and moments. Perimeter columns and flat plate floorconstruction complete the system. At mechanical floors, outrigger walls are provided to link the perimeter

columns to the interior wall system, allowing the perimeter columns to participate in the lateral load resistance of

the structure; hence, all of the vertical concrete is utilized to support both gravity and lateral loads. The result is a

tower that is extremely stiff laterally and torsionally. It is also a very efficient structure because the gravity load-resisting system has been used to maximize its use in resisting lateral loads also.




A high performance exterior cladding system will be employed to withstand the extremetemperatures during the summer months in Dubai. Primary materials include reflectiveglazing, aluminum and textured stainless steel spandrel panels and vertical polishedstainless steel tubular fins accentuating the height and slenderness of the tower.

As the building spirals in height, the wings set back to provide many different floor plates. The setbacks are

organized with the tower’s grid, such that the building stepping is accomplished by aligning columns above withwalls below to provide a smooth load path. As such, the tower does not contain any structural transfers. These

setbacks also have the advantage of providing a different width to the tower for each differing floor plate. This

stepping and shaping of the tower has the effect of “confusing” the wind. The upshot is that wind vortices never

get organized over the height of the building because at each new tier the wind encounters a different buildingshape.

Most of the tower is a reinforced concrete structure, except for the top, which consists of a structural steel spirewith a diagonally braced lateral system. High-performance concrete is utilized throughout. The concrete mix was

designed to provide a low-permeability yet high-durability concrete. Wall and column concrete strengths range

from C80 to C60 cube strength (11.6 kips per square inch (ksi) to 8.7 ksi cube strength), and contain portland

cement, fly ash, and local aggregates. The C80 concrete has a specified Young’s Elastic Modulus of 43,800

N/mm2 (6,350 ksi) at 90 days.

Structural analysis

The entire building structure was analyzed for gravity (including P-Delta analysis), wind, and seismic loadings

utilizing ETABS version 8.4, from Computers and Structures, Inc. The 3D analysis model consisted of the

reinforced concrete walls, link beams, slabs, raft, piles, and the spire structural steel system. Under lateral windloading, the building deflections are well below commonly used criteria. The dynamic analysis indicated the first

mode is lateral sidesway with a period of 11.3 seconds. The second mode is a perpendicular lateral sidesway

with a period of 10.2 seconds. Torsion is the fifth mode with a period of 4.3 seconds.




The Burj Dubai Tower pushes the limits on construction techniques and materialtechnology, including self-climbing formwork system, prefabricated wallreinforcement, specially modified cranes, high-speed high-capacity constructionhoists, and GPS monitoring systems.



Tower foundations

The tower foundations consist of a solid, 3.7-meter (12.1-foot) thick pile supported raft poured utilizing 12,500

cubic meters (m3) (16,350 cubic yards, yd3) of C50 cube strength (7.25-ksi) self-consolidating concrete

(SCC). The raft was constructed in four separate pours (three wings and the center core). Each raft pouroccurred during at least a 24-hour period. Reinforcement was typically spaced at 300 mm (12 inches) on center

in the raft, and arranged such that every tenth bar in each direction was omitted, resulting in a series of “pour

enhancement strips” throughout the raft; the intersections of these strips created 600-mm by 600-mm (24-inch

by 24-inch) openings at regular intervals, facilitating access and concrete placement. The tower raft is supportedby 194 bored cast-in-place piles. The piles are 1.5 m (5 feet) in diameter and approximately 43 m (141 feet)

long, with a capacity of 3,000 metric tonnes (3,300 tons) each. Each was pile load tested to 6,000 metric

tonnes (6,600 tons). The diameter and length of the piles represent the largest and longest piles conventionally

available in the region. Additionally, the 6,000-metric-tonne pile load test represented the largest magnitude pileload test performed to date within the region. The piles utilized C60 cube strength (8.7-ksi) SCC concrete,

placed by the tremie method utilizing polymer slurry. The friction piles are supported in the naturally cemented

calcisiltite/conglomeritic calcisiltite formations, developing an ultimate pile skin friction of 250 to 350 kPa (5.2 to7.3 ksf).

Wind engineering

For a building of this height and slenderness, wind forces and the resulting motions in the upper levels becomedominant factors in the structural design. An extensive program of wind tunnel tests and other studies were

undertaken by the wind tunnel consultant, RWDI, in its boundary layer wind tunnels in Guelph, Ontario, to

evaluate the effects of wind on building loading, behavior, and occupant comfort. Additionally, the wind tunnel

testing program was utilized as part of a process to shape the building to minimize wind effects. As mentionedabove, this process resulted in a substantial reduction in wind forces on the tower by confusing the wind — by

encouraging disorganized vortex shedding over the height of the tower. The wind tunnel testing program included

rigid-model force balance tests, a full aeroelastic model study, measurements of localized pressures, and

pedestrian wind environment studies. Wind statistics played an important role in relating the predicted levels of

response to return period. Extensive use was made of ground-based wind data, balloon data, and computersimulations employing Regional Atmospheric Modeling techniques to establish the wind regime at the upper

levels. Based on the results of the wind tunnel testing program, the predicted building motions are within the ISO

standard recommended values without the need for auxiliary damping.




On a clear day the tip of the spire can be seen by a person 95 km (60 miles) away. Aperson in the Observation Deck can see as far away as 80 km (50 miles), which is morethan twice the distance from Dubai to Al Ain.


An extensive program of wind tunnel tests and other studies resulted in a substantialreduction in wind forces on the tower by confusing the wind.

Construction methods and technology

The Burj Dubai Tower utilizes the latest advancements in construction techniques and material technology. The

walls are formed using Doka’s SKE 100 automatic self-climbing formwork system. The circular nose columns

are formed with circular steel forms, and the floor slabs are poured on MevaDec panel formwork. Wall

reinforcement is prefabricated on the ground to allow for fast placement. Three primary self-climbing Favco

tower cranes are located adjacent to the central core, with each continuing to various heights as required. The

cranes have been specially modified to be able to lift the extreme lengths of cable required, as well as 25-metric-



tonne (27.5-ton) payloads, at high speeds. High-speed (120-m/minute, 393-foot/minute), high-capacity (3,200-kg, 7,050-pound) construction hoists were used to transport workers and materials to the required heights.

Because of limitations of conventional surveying techniques, a specialized GPS monitoring system has been

developed to monitor the verticality of the structure.

The construction sequence for the structure has the central core and slabs being cast first, in three sections; the

wing walls and slabs follow behind; and the wing nose columns and slabs follow behind these. Concrete is

distributed to each wing utilizing concrete booms that are attached to the jump form system. Two of the largestconcrete pumps in the world were used to deliver concrete to heights over 600 m (1,968 feet) in a single stage.

A horizontal pumping trial was conducted prior to the start of the superstructure construction to ensure

pumpability of the concrete mixes.

Conclusion

Burj Dubai Tower has eclipsed all previous height records, and is the tallest structure ever built. It represents an

enormous collaboration and coordination effort of many individuals across all sectors of the building profession.

Conventional and cutting-edge technologies and building systems were utilized, developed, and further advancedto create this unprecedented structure, taking this building and the profession to literally new heights.

The Burj Dubai

Owner Emaar Properties PJSC, Dubai

Project manager Turner Construction International

Architect/Structural engineers/ MEP engineers Skidmore, Owings & Merrill LLP

Adopting architect and engineer/Field supervision Hyder Consulting Ltd.

General contractor Samsung/BeSix/Arabtec

Foundation contractor NASA Multiplex

By the Numbers: Burj Dubai

Size, shape, and type

Number of square feet: 3 million ft2 (280,000 m2)

Number of stories: 160+

Structural system types: concrete with structural steel spire

Foundation type: concrete raft on piles

Construction quantities

Concrete: 327,000 yds3 (250,000 m3)

This is equivalent to:

A solid cube of concrete 63 meters (207 feet) on a side,A sidewalk 2,065 kilometers long (1,283 miles),More than five times the volume of concrete used for the CN Tower in Canada,orThe weight of 110,000 elephants



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Rebar: 35,700 metric tonnes

Laid end to end this would extend over a quarter of the way around the world

Curtain wall: 83,600 m2 (20.7 acres) of glass and

27,900 m2 (6.8 acres) of metal; 111,500 m2 (27.5 acres) total

Equivalent to 17 soccer fields or 25 American football fields

Spotlight: Skidmore, Owings & Merrill LLC

Q&A with the SE

Skidmore, Owings & Merrill’s Structural Engineering Partner William F. Baker, P.E., S.E. (WB)discussed the Burj Dubai with Structural Engineer Editor Jennifer Goupil, P.E. (JG).

JG: You have alluded to the fact that the tower originally was conceived a few hundred feettaller than the current record holders, at what point did the design evolve to be 1,000 feettaller than today’s tallest buildings?

WB: Not even that much taller. The first design we tested was a few meters taller than thecurrent record holders. Now, actually — the final design — is several hundred meters taller

than current towers. But, there wasn’t really one event, no single epiphany or breakthrough. It was a series of processes.We started with an idea, then did some testing and analyzed data. At the same time the client kept changing what theywere trying to achieve. Our results from the first round of testing were not very good, so we went back to the boards. Atabout the third round of testing the data coming back was quite good. We probably kept re-massing the building for morethan a year. Stretching and fine tuning the shape, we tested multiple building shapes and structural properties along theway. In fact, our testing went on from May 2003 (the first test) through September 2005, which was the final aeroelastictest.

JG: How did you select the final concrete structural system?

WB: We did not have a contractor on board early in the design process. We communicated with our client and discussedoptions among our team. In fact, for a while we were carrying a composite option — concrete corewalls with compositesteel floor framing, etc. We carried two schemes along for a while, but ultimately we needed to choose one system tofinalize the design — our internal team decided to go with the all concrete scheme.

Later, as we interviewed contractors, we asked them if they wanted to change systems to the composite one and no onewanted to change; so we guessed right.

JG: From your perspective, what was the most challenging aspect of the structural design? How was it solved?

WB: The main difficulty was the shaping of the building. As I said earlier, the very first wind tunnel data revealed largeforces, large movements. So we didn’t panic…we went back to the massing. I remember sitting in my office with thearchitects: we would look at the data, then we’d look at the model, then we’d look at the data again, and look at the modelagain. Then we’d change the model and retest.

So the most challenging, yet most interesting, was engineering the shape of the building and understanding the wind tominimize the forces on the building. This was also the most rewarding part of the design work.

JG: When will the record-breaking height of the tower be revealed?

WB: It is not quite decided (laugh). The client has yet to commit to when he will reveal the height.

JG: When will the construction be complete?

WB: The owners will open the building in January of 2010; the building structure has been completed for some time.

JG: Aside from the structural challenges of a tower that is well over 160 stories tall, what were some of the buildingsystem challenges that you were involved with solving, such as elevators, stairs, egress, water distribution?

WB: One of the things I realized early on was that we had to tie the building together periodically with outriggers. Becausethe story heights were smaller, we needed three stories for outriggers. When we said that in one of the first coordinationmeetings, one of the architects actually said great then I can stack my elevator shafts with machine rooms below the

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elevator pits. We spent a long time working with the architects to compact the core. We went back and forth multiple timeswith the architects finally getting all the elevators and services inside a compact core. Ahmad Abdelrazaq was particularlytenacious in getting that core coordination to work.

JG: What did you learn from leading the structural engineering on the world’s tallest tower that was the mostfascinating to you?

WB: (Laugh) The Hitchhikers Guide to the Galaxy says don’t panic. As you come up with challenges and problems…don’tpanic, just work the problem. There were many challenges on this project, but the part that was so fascinating to me wasjust the fact that we were so successful. We worked hard to get the wind loads reduced. We’d reshaped the tower andretune the tower to change the periods and mode shapes. It was fascinating to me that the wind forces would drop a lot.The process was more successful than we ever imagined it would be.

JG: Had you done much of that with other tall towers? Reshaping?

WB: Yes, we did with a previous project, so we knew the shape was important. But on this project we really developed thatmethod. We discovered that we could not only design the building, but we could design the wind.

Firm Facts Established in 1936, Skidmore, Owings & Merrill LLP employs 869 design professionals in Chicago, New York, SanFrancisco, London, Washington D.C., Hong Kong, Shanghai, and Dubai. The firm’s diverse practice includes architecture,structural and civil engineering, interior design, urban design and planning, interior design, building services/MEP, digitaldesign, graphics, and industrial design. Markets served by the firm include residential, commercial and development,convention and exhibition, cultural, education, health and science, hospitality, infrastructure, sports, transportation, andcivic and government.

The firm has been recognized with many industry awards including The American Institute of Architects Architectural FirmAward 1962 and 1996; the American Concrete Institute Charles S. Whitney Award 2009; Fast Company magazinerecognized SOM as The Most Innovative Company in Architecture, and No. 32 on The World’s 50 Most InnovativeCompanies in 2009.

William F. Baker, P.E., S.E., is structural and civil engineering partner at Skidmore, Owings & Merrill

LLP in Chicago. James J. Pawlikowski, S.E., LEED AP, is an associate director at Skidmore, Owings &

Merrill LLP.

Structural design of Burj Khalifa

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