Burj Khalifa: Construction of the Tallest Structure in the World

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The Burj Khalifa is the world's tallest structure with a height of 828 m and has a total of 163 floors. Famous as a mixed-use development tower with a total floor area of 460,000 m², it includes residential, hotel, commercial, office, entertainment, shopping, and leisure establishments.

The idea behind the design of Burj Khalifa originated from the geometry of the indigenous desert flower of Saudi Arabia, while the design patterns of the tower personify Islamic architecture. The tower is constructed around a central core with three wings, and each wing is composed of four bays. On every seventh floor, one outer bay peels away as the structure spirals into the sky.

Unlike many tall towers with multi-layered floor plates, a Y-shaped floor plan was adopted for the Burj Khalifa to enlarge the view and provide tenants with plenty of natural light. Also, the Y-shaped plan was considered to reduce the effect of wind load on the building.

In addition, it has a podium around the base of the structure. It also includes four to six stories of underground parking garage. The tower is constructed on a 3.7 m thick raft foundation and supported by bored piles. The piles are 1.5 m in diameter, extending 50 m below the base of the raft foundation.

The crucial difficulties in designing the tower were encountered while carrying out a cost-effective foundation design, where the soil and rock conditions were poor, and a substantial amount of wind load had to be resisted. 

The structural and geotechnical features of the Burj Khalifa Tower, such as the geology of the area, foundation design, construction process, and pile load testing program have been discussed in this article.

1. Location of the Burj Khalifa

The Burj Khalifa Tower was opened to the public in January 2010. It is located in Dubai, United Arab Emirates. The figure shown below represents the location of the Burj Khalifa Tower. 

2. Geology of the Burj Khalifa Site

The following points describe the geology of the Burj Khalifa site:

1. The ground conditions were made up of a horizontally stratified subsurface profile, which was extremely variable and complicated due to the nature of deposition and the common hot-dry weather conditions.
2. Medium-thick to extremely loose granular silty sands (marine deposits) were present at a few meters below the base. The thickness of this strata is 4 m.
3. Silty sands are underlain by extremely weak to weak sandstone. This layer is interbedded with extremely weak cemented sand, fine-grained siltstone and weak to reasonably weak conglomerate. The thickness of this strata is 70 m.
4. Groundwater levels were typically high throughout the location. During excavations, the groundwater table was at around 2.5 m below ground level.
5. Unconfined compressive strength of granular silty sand was around 2 to 3 MPa, and that of the sandstone layer was around 1 to 3 MPa.
6. The geotechnical outcomes showed a potential capacity to destroy the stiffness of material under cyclic earthquake loading. However, after the installation of piles, the stiffness capacity of soil-pile interaction material was enough to resist cyclic earthquake loading.

3. Structural System

Lateral load resisting system and floor framing system are the two major components of the superstructure of the Burj Khalifa Tower and these systems are discussed below.

3.1 Lateral Load Resisting System

The lateral load resisting system of the tower consists of the ductile core walls made up of high-performance reinforced concrete. These walls were connected to the exterior reinforced concrete columns through a series of reinforced concrete shear wall panels. The following points describe the lateral load resisting system of the Burj Khalifa Tower:

1. The thickness of the core wall varies between 1300 mm to 500 mm.
2. Composite beams of reinforced concrete were used to connect the core walls of the tower, and these beams were 800 mm to 1100 mm thick.  
3. At some locations, the composite beams could not be provided due to limitations on depth. Therefore, at such locations, built steel beams were provided.
4. Width of both composite beam and steel beams was provided so that it matches the adjacent core wall width.
5. A very tall spire was provided at the top of the core wall. This spire was provided to make the structure the world's tallest tower in all categories.

3.2 Floor Framing System

The following points describe the floor framing system of the Burj Khalifa Tower:

1. Two-way reinforced concrete flat slabs were provided for both hotel and residential floors. The thickness of slab for flooring system is varying between 200 mm to 300 mm.
2. The spacing of slabs was kept at 9 m between the interior core wall and exterior columns.
3. At the tip of the tower, 225 mm to 250 mm thick two-way reinforced concrete flat slabs were provided.
4. However, within the interior core, flat slab with beams was provided to provide more lateral resistance.

4. Foundation System

Pile foundation was adopted to resist the vertical and lateral loads of the world's tallest structure. The following points describe the details of the foundation design of the Burj Khalifa Tower:

1. The tower was established on a 3.7 m thick raft supported on 194 high performances reinforced concrete bored piles. The diameter of piles was 1.5 m and piles were provided 50 m below the base of the raft.
2. The podium was established on a 0.65 m thick raft supported on 750 bored piles. The diameter of piles is 0.9 m and piles were extending 35 m below the base of the raft.
3. The reinforced concrete raft foundation was constructed with high-performance self-compacting concrete (SCC). A minimum 100 mm blinding slab was provided as a waterproofing membrane.
4. Polymer drilling fluid was used to construct the pile foundation. It was way more effective than the conventional bentonite drilling fluid because it enhanced the workability of the piles beyond expectations.
5. The maximum load of 35 MN was observed on the corners of piles. In contrast, a minimum load of 12-13 MN was observed on the center of the piles.
6. To restrict the lateral movement of the pile block, a factor of safety of 2 was adopted for both lateral and vertical load on the pile group.
7. Waterproofing members were provided at the bottom and sides of the raft foundation to protect it from water ingress.
8. The raft foundation bottom and all sides were protected with waterproofing membrane.
9. Tremie method was adopted for continuous pouring of concrete for piles and a 0.30 ratio of w/c was adopted for SCC. 
10. To protect the foundation system of Burj Khalifa Tower, a robust cathodic protection system was developed. This system provided safety against the chloride and sulphate attack from the soil at the site.
11. Marine deposit and silty sand soil were present till 3.5 m from the ground surface. Therefore, the possibility of the occurrence of liquefaction during the seismic event was high. Thus, a liquefaction assessment was carried out. However, the pile foundation was provided below the level of marine deposit and silty sand soil to make it safe.    

4.1 Pile load Testing Program

A static pile load test was performed in two stages. The first one consisted of loading on seven trail piles before the construction of the foundation. The second one consisted of loading on the eight working piles and it was performed during the construction of the foundation.

In addition, a total of ten piles were chosen for the dynamic pile load test. Also, a sonic integrity test was performed to check the vertical and lateral capacity of piles during the construction of the foundation.

The main goal of the pile load testing program was to develop a load-settlement response curve of piles and to validate the design assumptions. The following factors were studied during various pile load tests:  

1. Pile shaft length effect
2. Shaft grouting effect
3. Shaft diameter effect
4. Uplift (tension) loading effect
5. Lateral loading effect
6. Cyclic loading effect

Results of pile load testing are summarized below:

1. At working load, the factor of safety against bearing capacity failure was more than three. Thus, the tower was safe by a comfortable margin against bearing capacity failure.
2. Point load capacity of piles was more than the ultimate axial load capacity. However, the skin friction capacity of piles was fully mobilized above 30 m although significant skin friction capacity was available below 30 m.
3. The maximum settlement was within 70 mm for individual pile, which was way below the limit.
4. The effect of shaft grouting increased the skin friction capacity of the piles.
5. Under cyclic and lateral loading, the stiffness values were very high, thus it provided an excellent margin of safety.
6. Factor of safety against uplift was 2 due to which the uplift pressure was influencing the compressive capacity of the piles.

5. Construction of Burj Khalifa Tower 

For the construction of the tower, firstly pile and raft foundation works were completed by February 2005. After that, the construction of the superstructure of the tower started in April 2005, and its tower was completely constructed to its desired position in January 2009.

The following technologies and strategies were implemented to construct the tower within the timeframe:

1. A unique approach of a 3-day cycle was adopted for structural works.
2. A transportation system with a huge capacity of equipment and optimum building materials was adopted.
3. Optimum formwork system was provided to meet the requirement of tower construction along with the height of the tower.
4. Logistic plans were developed during the course of tower construction.

5.1 Planning for the Concrete Work

For the successful construction of the tower, the main focus was on the concrete testing and quality program. These programs were commenced soon after the development of mix design criteria and continued until the last phase of the construction process. The following points describe the testing regimes included for the construction of the Burj Khalifa Tower:

1. All the mechanical properties, such as modulus of elasticity, tensile strength, and compressive strength of concrete were calculated.
2. Durability tests were performed. These tests included initial and 30-minute surface absorption test.
3. Set up for creep and shrinkage test was developed for different types of concrete mix designs.
4. Permeability test such as rapid chloride was performed.
5. Heat of hydration test was performed. This test consists of cube analysis and full scale set up for measuring the effect of the heat of hydration on large-sized concrete elements that have a dimension greater than 1.0 m.
6. Pump simulation tests were performed so that the concrete pumpability to a large distance could be achieved. 

In short, all these tests were conducted to confirm the construction sequence of large-sized elements and to develop curing plans while considering the daily and seasonal temperature fluctuations.

5.2 Technologies Used to Achieve 3-Day Cycle

To construct the tower of such magnitude within a very tight schedule. a 3-day cycle program was developed for concrete work. The following points describe the construction technologies used to accomplish the 3-day cycle program for concrete work:

1. Auto Climbing formwork system (ACS) was used for construction at greater heights.
2. High-performance concrete was used to provide high durability requirements, high modulus, high strength, and pumping requirements.
3. While keeping the labors requirement as low as possible, a simple drop head formwork system was developed to provide a semi-automatic dismantling and assembling process of formwork.
4. Rebar pre-fabrication was used for a faster construction process and to reduce human error in rebar fabrication.

5.3 Superstructure Construction Sequence 

The process of superstructure construction and ACS is depicted in Figure-9. The ACS work was divided into three segments. The first segment included the construction of the center core wall, and the second segment included the construction of the wing wall. The third segment included the construction of three tower wings. The following points describe the construction sequence of the superstructure of the Burj Khalifa Tower:

1. Firstly, the main central core walls were constructed, followed by the construction of the center core slab.
2. After that, the wing wall was constructed in continuation with the construction of the wing flat slab.
3. Further, nose columns were constructed along with the construction of flat slab at the nose area.
4. Besides, the main central core walls were tied to the nose columns by a series of braced walls at each mechanical level.
5. The braced walls were constructed using structural steel members because the reinforcing bars were making the connections congested. Therefore, to reduce the construction time and to achieve more joint rigidity, structural steel members were used.

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