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.

 

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 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.

 

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 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).

 

Wind Engineering

 

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.

 

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 advanced to create this unprecedented structure, taking this building and the profession to literally new heights.