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 be able 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 just outside 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 it also 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.
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 a central core. As the tower rises, one wing at each tier sets back in a spiraling pattern, further emphasizing its height. 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 the central 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 the occupied 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 terms of its functionality, structural system, and response to wind, while still maintaining the integrity of the initial design concept.
Structural system description
The tower’s Y-shaped floor plan not only has aesthetic and functional advantages, but also is ideal for providing a 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 the others 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 each wing, 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 floor construction 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.
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 with walls 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 building shape.
Most of the tower is a reinforced concrete structure, except for the top, which consists of a structural steel spire with 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.
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 wind loading, 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 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 pour occurred 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 supported by 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 pile load 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 to 7.3 ksf).
For a building of this height and slenderness, wind forces and the resulting motions in the upper levels become dominant 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 mentioned above, 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 computer simulations 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.
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 largest concrete 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.
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 advanced to create this unprecedented structure, taking this building and the profession to literally new heights.
| The Burj Dubai
Architect/Structural engineers/ MEP engineers
Adopting architect and engineer/Field supervision
|By the Numbers:||Burj Dubai|
|Size, shape, and type|
|3 million ft2 (280,000 m2)|
|concrete with structural steel spire|
|concrete raft on piles|
|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, or
The weight of 110,000 elephants
|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 feet taller than the current record holders, at what point did the design evolve to be 1,000 feet taller than today’s tallest buildings?
WB: Not even that much taller. The first design we tested was a few meters taller than the current 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 they were trying to achieve. Our results from the first round of testing were not very good, so we went back to the boards. At about the third round of testing the data coming back was quite good. We probably kept re-massing the building for more than a year. Stretching and fine tuning the shape, we tested multiple building shapes and structural properties along the way. In fact, our testing went on from May 2003 (the first test) through September 2005, which was the final aeroelastic test.
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 discussed options among our team. In fact, for a while we were carrying a composite option — concrete corewalls with composite steel floor framing, etc. We carried two schemes along for a while, but ultimately we needed to choose one system to finalize 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 one wanted 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 large forces, large movements. So we didn’t panic…we went back to the massing. I remember sitting in my office with the architects: 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 model again. 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 to minimize 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 building system 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. Because the story heights were smaller, we needed three stories for outriggers. When we said that in one of the first coordination meetings, one of the architects actually said great then I can stack my elevator shafts with machine rooms below the elevator pits. We spent a long time working with the architects to compact the core. We went back and forth multiple times with the architects finally getting all the elevators and services inside a compact core. Ahmad Abdelrazaq was particularly tenacious 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 most fascinating to you?
WB: (Laugh) The Hitchhikers Guide to the Galaxy says don’t panic. As you come up with challenges and problems…don’t panic, just work the problem. There were many challenges on this project, but the part that was so fascinating to me was just the fact that we were so successful. We worked hard to get the wind loads reduced. We’d reshaped the tower and retune 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 that method. We discovered that we could not only design the building, but we could design the wind.
The firm has been recognized with many industry awards including The American Institute of Architects Architectural Firm Award 1962 and 1996; the American Concrete Institute Charles S. Whitney Award 2009; Fast Company magazine recognized SOM as The Most Innovative Company in Architecture, and No. 32 on The World’s 50 Most Innovative Companies 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.