Construction of Burj Khalifa?

August 30, 2022

ABOUT THE AUTHOR

Namrata Karande was working as a Product developer : Civil Engineer at Lesics Engineers Pvt. Ltd. Namrata has worked on projects such as Footing, Hoover dam and Burj khalifa.


In the previous article I had explained the basic design concepts of Burj Khalifa. Along with some interesting facts.In this article I will explain why the Burj Khalifa has a unique shape, its lightning protection, how they built this building upto 830 meter on loose sandy soil. So let's start with its unique shape.

Why is the shape of the Burj Khalifa Asymmetrically stepped?

Have you ever wondered why the Burj Khalifa is an asymmetrical design? Consider two designs - One with symmetrical steps and another with asymmetrical steps.

Fig 1a : Imaginary shape vs real shape

The reason behind the asymmetrical design is vortex shedding(refer Fig 1b). This phenomenon occurs when the wind passes around a structure and leads to very beautiful and dangerous vibrations in the building and can collapse the building.

Fig 1b : Vortex shedding

This is dangerous because the vortices are fluctuating. This generates fluctuating force on this building. If the natural frequency of vibration of this building is the same as the frequency of these fluctuations in force, the building will violently oscillate. a phenomenon called resonance. These oscillations will continue to increase and eventually the Burj Khalifa may fail completely.

How does the Burj Khalifa overcome vortex shedding?

To overcome the issue of vortex shedding, engineers came up with an architectural innovation. A spiralling Burj Khalifa (refer Fig 2a). A spiral shape is the best solution to kill vortex shedding; it just confuses the wind, at each step its cross section shape varies.

Fig 2a : Burj Khalifa in spiral shape

These innovations are the reason the Burj Khalifa has successfully reached 830 meters. Do you know an interesting fact? Initially The Burj Khalifa was only designed to reach 518 meters. An additional 312 metre that’s almost as tall as the Eiffel Tower was added during construction(refer Fig 2b).

Fig 2b: Height of Burj Khalifa

How does the Burj Khalifa stand on loose sandy soil?

Have you ever wondered how such a heavy structure is able to stand on the loose sandy soil of Arabia without sinking drastically? The engineers of this building couldn’t find any hard strata in this area even after going 140 meters below the ground(refer fig 3a). The sedimentary rocks present at very bottom end, were weak. So the engineers faced two challenges: one was the differential settlement and the second was poor load-bearing capacity of the ground.

1.Differential settlement - Differential settlement is when a building sinks only from one side i.e uneven sinking. This forms deadly cracks on buildings!

Fig 3a : Weak base at Burj Khalifa location

To tackle this issue they did their analysis on site and used a thick Raft as a base. The raft was made of a slab of thick, high-performance RCC. They attached it to the hexagonal core, I sections, and columns together(refer fig 3b). This raft distributes the weight of the building onto a larger area of sand equally, and thus resists differential settlement.

Fig 3b : Raft foundation of Burj Khalifa

2.Load bearing capacity - This is the ability of the ground beneath the structure to withstand its weight without sinking.

Coming back to our foundation design, one big problem is still unsolved. How will the weak sand and rocks bear such a huge mass? This raft along with Burj Khalifa will just sink. Engineers came up with a solution: Skin friction. They used multiple RCC piles connected at the bottom of the raft(refer Fig 4a). Frictional force between these pile surfaces and weak but stiff sedimentary rocks will resist the building load perfectly.(refer fig 4b) This is how the Tallest building in the world stands strong on weak soil! .

Fig 4a : Raft and Pile foundation
Fig 4b : frictional force between piles and soil

Interesting fact about the Burj Khalifa's cross-sectional shape?

Did you notice the beautiful flower-like petals just above the base of the building? This structural design, especially the petals, was inspired by one of Dubai’s most widely cultivated flowers: the spider lily(refer fig 5a).

Fig 5a : The shape of building inspired by Spider Lily

How The Burj Khalifa Is Dubai's Lighting Rod?

These lightning strikes on the Burj Khalifa are fascinating. At the very top of this building, there is a small, fantastic device that prevents the Burj Khalifa from burning down. That device is a lightning arrester.

This dome is connected to a sensor equipment which is powered by a tiny battery. As soon as it detects a super-charged cloud nearby, the dome immediately generates opposite charges on its surface to attract the lightning towards it(refer fig 5b). The electron flow of lightning is then passed through the outer column’s steel structure and dissipated into the earth pits via the steel inside the foundation piles(refer fig 5c).

Fig 5b : Lightning arrester
Fig 5c: lightning dissipated into the earth

The electrons travel only through the external columns due to the skin effect. Burj Khalifa is Dubai’s lighting rod. Its lightning arrester collects the lightning strokes of almost the entire Dubai.

Details of concrete using in Burj Khalifa

How did they lift the concrete to this height in a warmer dubai? Due to this, the most powerful concrete pumps would have hardened before reaching the destination. Instead, the team used special grades of concrete called C50 and C80, mixed with cold ice as they pumped it (refer fig 6). This work of pouring the concrete into molds was done at night (normal temperatures). The team completed one floor every week; the entire project took six years to complete.

Fig 6 : Construction details

Is it possible to build a taller building than Burj Khalifa?

Do you think the height of Burj Khalifa will surpass any other building? As we knew, the design challenges increase as the building gets taller. If the engineers came up with some unique ideas for the construction work and if the land and air conditions are right then they can also build a tall building but remember it is not an easy task.

I hope you understood the amazing construction details of this massive structure and why the Burj Khalifa is so tall and strong. I am sure you found both Article 1 and Article 2 informative.

Thanks for reading!