Burj Dubai – World’s Tallest Building

Greg Sang1

INTRODUCTION

The Burj Dubai Tower, currently under construction in Dubai, UAE, will be the tallest structure in the world when it opens in 2009. The Tower will be over 700m tall, making it the tallest by a significant margin. It is the centerpiece of the Burj Dubai Downtown, a nearly 4,000,000m

2, AED 73 billion (US$20 billion)

development currently being built by Emaar Properties PJSC of Dubai (Figure 1). The Burj Dubai Tower part of the development consists of the Tower itself, as well as a 2-story basement and 3-story podium, and separate 6-story Office Annex and 3-story Pool Annex. At 280,000m

2, the Tower is a mixed use reinforced

concrete structure containing mainly residential units, but also includes a hotel designed by Giorgio Armani, serviced apartments, boutique offices, an observatory, and communication floors. The 180,000m

2 reinforced

concrete basement and podium contains mainly underground parking, but also houses building services and hotel amenities. The Burj Dubai Tower architects and engineers are Skidmore, Owings & Merrill LLP of Chicago, USA. Turner International is the project and construction manager, with the construction team led by Samsung Corporation of South Korea. Hyder Consulting Middle East are supervising the construction. With the Burj Dubai, Emaar Properties PJSC has stated that their goal is not simply to be the world’s tallest building, but also to embody humanity’s highest aspirations.

1

Assistant Director – Projects, Emaar Properties PJSC, Dubai, UAE

Figure 1: Emaar Properties PJSC Burj Dubai Downtown Development

WORLD’S TALLEST BUILDING

From the outset, it has been intended that the Burj Dubai be not only the Worlds’ Tallest Building, but the world’s tallest freestanding manmade structure. The official arbiter of height is the Council on Tall Buildings and Urban Habitat (CTBUH) founded at Lehigh University in Bethlehem, Pennsylvania, and currently housed at the Illinois Institute of Technology in Chicago, Illinois. The CTBUH measures the height of buildings using four categories. The categories and current record holders are as follows:

Highest Occupied Floor: Taipei 101 439m

Top of Roof Taipei 101 449m

Top of Structure Taipei101 509m

Top of Pinnacle, Mast, Sears Tower 527m Antenna or Flagpole

Although the final height of the Tower is a well-guarded secret, Burj Dubai will be the tallest by a significant amount in all four categories (Figure 2).

ARCHITECTURAL DESIGN

The context of the Burj Dubai being in the city of Dubai, in the Middle East, drove the inspiration for the building form to incorporate cultural, historical, and organic influences particular to the region. The influences of the onion domes and pointed arches in traditional buildings, spiral imagery in Middle Eastern architecture, together with the structure of an indigenous desert flower with its central structure and surrounding petals, resulted in the tri-axial geometry of the Burj with spiral reduction with height. In plan, the Tower is Y-shaped (Figure 3). As the Tower rises, the wings set back in a spiral pattern, emphasizing its height, until it reaches its central shaft at which point the shaft peels away to reveal the single spire (Figure 4). The Y-shape plan is ideal for residential and hotel usage, with the wings allowing maximum outward views and inward natural light. The central core contains all of the elevatoring and mechanical risers. The Tower is serviced by five separate mechanical zones, located approximately 30 floors apart over the height of the building. Above the occupied concrete portion of the building is the structural steel spire, housing communications and further mechanical floors. The architects and engineers worked hand in hand to develop the building form and the structural system, resulting in a tower which efficiently manages its response to the wind, while maintaining the integrity of the design concept.

Figure 2: Lineup diagram of several of the World’s Tallest Buildings

STRUCTURAL SYSTEM DESCRIPTION With wind being the critical factor in the design of super-tall buildings, the Tower was shaped so as to minimize the wind forces on the building. Structural simplicity and constructability were also key drivers of the design. The resulting structural system can be referred to as a “buttressed core”, with each of the three wings buttressing the other two via a six-sided central core (Figure 3). Constructed utilizing high performance concrete, corridor walls extend from the central core, down the axis of each wing, with hammerhead walls at the end of each corridor. Columns are located at the building perimeter and the tip of each wing, with the floor system consisting of flat plate construction. The corridor walls act as webs to resist shear, with the hammerhead walls acting as flanges to provide flexural stiffness. The closed hexagonal core provides an extremely stiff torsional restraint. At the mechanical zones, outrigger walls fully engage all perimeter columns, thus allowing the columns to participate in the resistance of lateral loads, as well as redistributing the gravity loads. The gravity load redistribution balances the stress levels in the vertical elements and minimizes the effects of differential shortening, an especially significant factor in this super-tall building. The building setbacks are located at structural bays and all vertical elements are aligned, negating the need for any transfer structures. The result is a very elegant and efficient structure which utilizes the gravity load resisting system to effectively assist in the resistance of lateral loads.

Figure 4: Tower Elevation Figure 3: Typical Floor Plan

WIND ENGINEERING

The shaping of the Tower has been strongly influenced by wind. The final design of the Y-shaped floor plan with spiraling setbacks creates a building with 27 different floor plates. The advantage of the constantly changing building form is to “confuse the wind” – to not allow the wind vortices to get organized, as each new tier presents a different building shape to the wind with a resultant different frequency of wind vortex shedding. The Tower has also been oriented to optimize its performance in the wind, as a result of the analysis of the wind climate data and the wind tunnel testing. As wind forces and the resulting building motions dominate the structural design, an extensive program of over 40 wind tunnel tests was carried out by Rowan Williams Davies and Irwin Inc. (RWDI) of Guelph, Ontario, Canada, with peer reviews carried out by the Boundary Layer Wind Tunnel Laboratory at the University of Western Ontario. The testing program included rigid-model force balance tests, full aeroelastic model studies, cladding pressure studies, and pedestrian wind studies. Models were typically 1:500 scale with 1:250 scale used for the pedestrian wind studies to test various wind mitigation devices at sensitive areas (Figures 5, 6, 7).

A series of force balance tests were used throughout the design phase to refine and confirm the Tower’s geometry. As the building’s shape was finalized, more accurate aeroelastic models were utilized to determine the wind-induced forces and motions on the building. The integration of the architectural and structural systems allowed the designers to significantly reduce the wind loads and resultant accelerations on the Tower. As a result, the building’s accelerations satisfy the comfort criteria without the need for any supplemental damping devices.

Figure 6: Aeroelastic Model in RWDI Wind Tunnel Figure 7: Cladding Pressure Model (at RWDI)

Figure 5: Smoke test of 1:250 scale model to evaluate wind behavior at upper level exterior terrace (at RWDI)

FOUNDATIONS AND SITE CONDITIONS The foundation system for the Burj Dubai consists of a 3700mm thick reinforced concrete raft supported by 194 reinforced concrete bored piles. The piles are 1500mm diameter, and are generally founded around 50m in the cemented calcisiltite / conglomeritic calcisiltie rock subgrade. The pile load test program implemented the largest pile test ever performed in Dubai, with a 6000 tonne test load (Figure 8). The piles were cast using C60 self consolidating concrete placed by tremie in a polymer slurry. The raft was C50 self consolidating concrete, and was poured in four sections - one under the central core, and one under each of the three wings (Figure 9). Reinforcement was typically at 300mm spacing, and arranged such that every 10

th bar in each

direction was omitted, resulting in a series of “pour enhancement strips” throughout the raft at which 600mm x 600mm openings at regular intervals facilitated access and concrete placement.

Due to the aggressive conditions present due to the extremely corrosive ground water, a rigorous program of anti-corrosion measures was required to ensure the durability of the foundations for the Tower’s 100 year design life. Measures implemented included specialized waterproofing systems, increased concrete cover, the addition of corrosion inhibitors to the concrete mix, stringent crack control design criteria, and a cathodic protection system.

Figure 9: Tower Raft Foundation under construction

Figure 8: Tower Pile load test

CONSTRUCTION The Burj Dubai utilizes the latest advancements in construction techniques and material technology. The walls are formed using Doka’s SKE 100 automatic self-climbing formwork system (Figure 10). The circular nose columns are formed with steel forms, and the floor slabs are poured on MevaDec formwork. Wall reinforcement is prefabricated on the ground in 8m sections to allow for fast placement for the 3-day cycle. The construction sequence for the structure has the central core and slabs being cast first, in three sections, the wing walls and slabs follow around 10 stories behind, and the wing nose columns and slabs around 10 stories behind again (Figure 11). The three self-climbing Favco tower cranes are located in the central core and are jumped every 4 stories. The cranes have been specially modified to be able to lift the extreme lengths of cable required, as well as 25 tonne payloads, at high speeds. High-speed (120m/min.) and high-capacity (3200kg) hoists are used to solve the logistics problems of moving men and materials associated with super-high-rise construction. Due to the limitations of conventional surveying techniques, a special GPS monitoring system has been developed to monitor the verticality of the structure. Concrete is pumped via specially developed Putzmeister pumps, able to pump to heights of 600m and generate 350 bar pressure, and is placed by four placing booms, one in the core and one in each of the wings.

CONCRETE TECHNOLOGY The groundwater in which the Burj Dubai substructure is constructed is particularly severe, with chloride concentrations of up to 4.5%, and sulfates of up to 0.6%. Accordingly, the primary consideration in designing the piles and raft foundation was durability. The concrete mix for the piles was a 60 MPa mix based on a triple blend with 25% fly ash, 7% silica fume, and a water cement ratio of 0.32. The concrete was also designed as a fully self consolidating concrete, incorporating a viscosity modifying admixture with a slump flow of 675 +/- 75mm to limit the possibility of defects during construction. The Tower raft is 3.7m thick, and therefore, in addition to durability, limiting peak temperature was an important consideration. The 50 MPa raft mix incorporated 40% fly ash and a water cement ratio of 0.34. The Tower piles and raft are also protected by an impressed current cathodic protection system. The design of the concrete for the vertical elements is determined by the requirements for a compressive strength of 10 MPa at 10 hours to permit the 3-day construction cycle and a design strength / modulus of 80 MPa / 44 GPa, as well as ensuring adequate pumpability and workability. The ambient conditions in Dubai vary from a cool winter to an extremely hot summer, with maximum temperatures occasionally exceeding 50º C. To accommodate the different rates of strength development and workability loss, the dosage and retardation level is adjusted for the different seasons.

Figure 10: Self-climbing formwork system

Figure 11: Tower construction photo

Ensuring pumpability to reach the world record heights up to 600m is probably the most difficult concrete design issue, particularly considering the high summer temperatures. Four separate basic mixes have been developed to enable reduced pumping pressure as the building gets higher. A 600m horizontal pumping trial was conducted in February 2005 to determine the pumpability of these mixes and establish the friction coefficients (Figure 12). The current concrete mix contains 13% fly ash and 10% silica fume with a maximum aggregate size of 20mm. The mix is virtually self consolidating with an average slump flow of approximately 600mm, and will be used until the pumping pressure exceeds approximately 200 bar. It is envisaged to change to a mix containing 14mm maximum aggregate size and 20% fly ash with full self consolidating characteristics while maintaining the required 80 MPa. Above level 127, the structural requirement reduces to 60 MPa, and a mix containing 10mm maximum aggregate may be used. Extremely high levels of quality control will be required to ensure pumpability up to the highest concrete floor, particularly considering the ambient temperatures. The pumps on site include two of the largest in the world, capable of generating pumping pressure up to a massive 350 bars through high pressure 150mm pipeline.

CONCLUSION When completed, the Burj Dubai Tower will be the world’s tallest structure by a wide margin. It represents a significant achievement in terms of utilizing the latest design, material, and construction technology and methods, in order to provide an efficient, rational structure to rise to heights never before seen.

PROJECT TEAM

Owner Emaar Properties PJSC

Project Manager Turner Construction International

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

Architect and Engineer of Record / Field Supervision Hyder Consulting Ltd.

General Contractor Samsung Corporation

Foundation Contractor NASA Multiplex

Figure 12: Concrete pumping system test