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What Are the Key Aspects in the Construction and Design of the Burj Dubai Tower?

In: Other Topics

Submitted By milkycat
Words 2012
Pages 9
The main reason The Burj Dubai tower works and allows its occupants to be comfortable lies in its structural design to deal with the main problems such as the wind load, the physical strength of the building and of course the elevator design needed for such a large structure. The Burj Dubai, situated in Dubai, will soon be the tallest building in the world, bypassing the previous record holder, the Taipei 101 by almost 290 metres giving it an astonishing 800 metre height. With over 2700 feet, designing and constructing such a vast building has given significant challenges to engineers and designers alike. Since the building can hold up to 35,000 people at once, it is essential that it is strong, comfortable and able to move people up and down it at a steady rate. This essay will explore how the designers and engineers went about solving these issues.
One of the key problems the engineers had to overcome was effect of the wind load and how it would affect the building motions due to the extraordinary height of the tower. Wind load is something that must be taken into account for all buildings, but obviously the wind velocity increases the higher you go and since the structure is over 800 metres tall, there is guaranteed to be a large wind force acting on the building. Three different engineering firms: Skidmore, Owings and Merrill who were all working on the project, hired the RWDI (Rowan Williams Davies & Irwin Inc.) a wind and environmental engineering firm to conduct wind studies on the Burj Dubai’s design. The RWDI had worked on a number of previous projects such as the Petronas Towers and the Taipei 101 and its facilities included four boundary wind tunnels. Their job was to minimize the effects of the wind load and building motion of the tower.
Wind around an extremely tall skyscraper can result in the wind load changing direction or force and can cause instability to the structure as well as large building motions. The RWDI also needed to test the wind load on the exterior cladding of the building. When wind hits the curtain wall (a façade that does not carry any load except its own, and is designed to resist air infiltration) it applies negative or positive suctioning pressures to the building’s edges. So as a start the Burj Dubai’s curtain walls had to be designed to withstand the positive and negative pressures exerted by the wind to keep the building stable. More importantly though, is that wind pressures on the surface of buildings are greatly influenced by the shape of the building itself. Designers had come up with a unique shape to reduce the wind forces on the tower and maintain a simple structure. The result was a concrete core design with three main wings each with their own perimeter columns making it extremely stiff. Each of the columns goes up in a spiral stepping pattern up the building just like a spiral flight of stairs so that its width actually changes at each setback.

The advantage of this was to literally weaken the wind; the wind vortexes could never organise themselves because at each new tier they would encounter a different building shape. However there were still many tests to be taken to determine the wind effects on the main structure. The wind tunnel program the RWDI used, designed to minimize these effects consisted of model force balance tests, an aero elastic model study, measurements of local wind pressures and pedestrian wind environment studies. All of the studies used models at a scale of mostly 1:500 except the pedestrian wind studies where a larger scale of 1:250 was used. The first tests were taken very early in the design in order to find out the tower’s dynamic response and overall effective wind load distributions.
For the Burj Dubai, the results of the first tests were used early for the structural design and allowed studies to be conducted on the effects of varying the tower’s stiffness and mass distribution. The studies showed that the building has six major wind directions. Three of these directions are when the wind blows directly onto a wing or “nose” (Nose A, Nose B and Nose C). The other three directions are when the wind blows in between two wings; these were called “tail” directions (Tail A, Tail B and Tail C).

More importantly it was noticed that the force for different wind directions showed less excitation in the frequency range for wind loads blowing on the end of a wing or nose. In comparison the force indicated more excitation in the frequency range for wind loads blowing onto the tail, between the wings. The reason for this difference in excitation was mainly due to the wings dividing the wind load when it struck. Their shape meant that the wind could not hit a large surface area and create a large force unlike with the tail directions where the wind could hit at maximum power. This became a key observation when constructing the Burj Dubai as the direction of the tower had to be fitted so that the frequent strong winds would hit the tail of the tower and the weaker winds would hit the nose. This made sure that on average the stronger wind loads would cause small excitation to the structure and reduce the building motions.

As the tower evolved the wind tests varied as the model was subjected to wind loads hitting it at different directions. They were usually directed at each of the three wings in turn starting with nose A and finishing with nose C. After each wind test, all the collected data would be analyzed with respect to the structure and every new found detail would be taken into account. The building itself would then be reshaped to minimize all the different wind effects they discovered. So throughout all the testing the number of steps changed as well as the shape of three wings. This meant that the testing continued during the construction so that the building could be modified with each new data analysis. This whole process caused a significant reduction in the wind load on the tower, the final design being able to weaken the wind with its uneven shape. Nearing the last tests, far more accurate aero elastic models were being used which could completely mirror the flexibility, mass and stiffness of the real building. These were able to display the major swaying motions and take measurements of the moments from the base and, because of stair pattern, other higher levels as well.

Some of the tests also had to be carried out for the comfort of pedestrians at ground level. For this, two aspects of wind load had to be considered: the mechanical effect of the wind and thermal which had to consider air temperature, humidity, sun radiation and wind velocity. This was mainly dealt with by combining wind speed measurements from the tunnel tests with the local climate information. It was also important to model things that might break the wind such as trees or other buildings inside the tunnel tests. This was not an issue on the other parts of the building as no other structure was as tall as that. Taking the results from the test led to the need to add in shade structures in order to avoid strong solar radiation impact on the ground level. These shade structures were set up and designed at the bottom. This was the final wind issue needed to be solved on the Burj Dubai and no more wind tunnel tests were needed afterwards.

Another important aspect that the engineers needed to deal with was the material design used for construction. Because of its vast size, the Burj Dubai would require plenty of material and thus gain plenty if weight. Engineers needed to make sure the building could support its own weight by using strong materials, but at the same time distribute them evenly so the weight load didn’t get too big. The centre walls were made of reinforced concrete which provided torsional resistance to the building. These centre hexagonal walls are also supported by the wing walls from the outside which act as ropes to resist any moments from the wind load. Essentially whichever direction the tower might want to move in, it is resisted by the full strength of the wings in the opposite direction. In addition to this, stabilizing strips of wood (outriggers) were placed along the floors at the bottom of each column, thus allowing the columns to actually participate in the load resistance on the side. This was a very clever idea as all of the vertical concrete on the columns ended up supporting the side and gravity loads. The engineers were actually using the building’s own weight to stabilize it. The concrete walls themselves were made of a specific strength which ranged from C80 to C60 and used cement and fly ash. The main concrete used on the lower level of the structure was C80 as it is stronger and has a large Young’s modulus of approximately 4.38×〖10〗^10 Pascals.

Another key problem that the engineers faced was the effects of creep and shrinkage on the concrete being utilized, creep being the technical term for time dependent deformation that occurs in all concrete when it is subjected to a load. To prevent this, the thickness of the walls and size of the columns had to be fine tuned. In addition, to prevent the effects of column shortening between the perimeter columns and interior walls (due to creep), the perimeter columns were sized to the point where their own self gravity load was equal to the load on the interior walls. This meant there was no resultant force acting between them so the effects of creep would be reduced. To tackle the effect on the whole building, five sets of outriggers where scattered up the structure, tying all the main vertical gravity load elements together and therefore making certain that the gravity stresses are uniform and so creep effects are minimized. What also had to be considered was that the rate of shrinkage depends on the thickness of the wall: thinner walls would shrink faster than thicker ones. Thus in order to keep the load uniform, the perimeter column thickness and interior wall thickness had to be the same (600 mm) so they would shrink at the same rate and keep the stresses even.

The design of the concrete for all vertical elements is mainly determined by the requirements for a certain compressive strength. In the case of the Burj Dubai this was around 10Mega pascals in addition to the adequate pump ability which had to be tested as well. In fact the concrete tests showed that the real concrete utilized was actually much stronger than the compressive strength required. What became a main problem however was ensuring the pump ability to reach world record heights most significantly at such high temperatures that can reach up to 50˚C. This was probably the most difficult concrete design issue and led to four basic mixes being developed to allow minimized pumping pressure as the height increases. In February 2005 a horizontal pumping test was conducted experiencing pressure loss equivalent to a height of 600 metres, to establish the pump ability of the mixes. The current mix consists now of 10% silica fume and 13% fly ash and has an average drop of roughly 600 mm. This mix could be used perfectly until the pumping pressure exceeded 200 bar. The height of the building meant that very high levels of quality control were be needed to make sure pump ability continued to the highest concrete floor. For this task the Putzmeister pumps were used which were two of the largest in the world and capable of pumping pressure up to 350 bars. These were easily the best suited for the job and overcame the initial problems of pumping concrete at such height.

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