18 Jan How was the Burj Khalifa, the world’s tallest skyscraper, built?
Dubai is the fastest growing city in the world. Thousands of workers, on construction sites with hundreds of cranes and machines, work tirelessly day and night, to help build the most dazzling city in the world.
It is in Dubai, not far from the desert, that we find the highest structure on earth: the Burj Khalifa (formerly Burj Dubai)
At a height of almost one kilometre (829.8m), the Burj Khalifa is a veritable city in the sky and a true testament to modern engineering.
However, it owes much of its success to the many ingenious technical solutions developed during the construction of other historical giants in the world of skyscrapers.
In order to understand how the Burj Khalifa could reach such dizzying heights, we’ll need to go back in time to see how its forerunners were built.
In the 19th century, the builders of the Equitable Life Building in New York set out to build a 5-storey (43m) building with easy access to the upper floors.
Stage 1: Mobility
At the time people were not willing to climb more than two or three floors on foot, and the elevators of the day were dangerous to say the least.
Elisha Graves Otis (1811-1861), a mechanic from Vermont invented a safety system to automatically bring elevators to a halt if the hoisting rope were to break. He presented his project at the 1854 New York World’s Fairand this would give rise to the fully automated elevator with safety features built in.
In 1870, the Equitable Life Building in New York, became the very first to have an elevator with this safety feature. Before the advent of safer elevators, the first floors of buildings were understandably very popular. But the advent of this new technology would have a profound impact on real-estate economics. Equitable Life had its offices on the lower floors and rented out the upper floors at premium rates. With better light and air quality, and far from the noisy streets below, the higher levels were suddenly much sought after.
But how far we have come since! Equitable Life had 7 floors, whereas the Burj Khalifa has no less than 162! And it can accommodate up to 35,000 people. To cope with this huge number of people, it has 53 elevators. Some of these have speeds in excess of 36 km/h, which makes them capable of climbing 124 floors in less than 50 seconds! Some of them can even fit up to 46 passengers at a time!
Stage 2: the materials
When the Monadnock Building in Chicago opens its doors in 1893, it was the largest office building in the world.
To support the weight of this stone building 16 stories high, the walls at its base were nearly 2 meters thick. Its extreme weight caused it to sink half a meter into the ground.
When the architect Daniel Burnham planned the construction of the 22-story (87m) triangular-shaped Flat Iron Building in New York he knew that stone, which was too heavy and expensive, would not do the trick.
Burnham went on to combine steel beams and columns to make a skeletal structure that would be more robust and more lightweight than anything seen before. The only stone used on the building would be on the façade itself.
As for the Burj Khalifa its internal structure combines the best qualities of both concrete and steel. More than 39,000 tons of steel rebar were used in its construction (the equivalent amount of steel used in 2,815 Airbus A380s), but in an ingenious way, since the steel skeleton is completely encased in concrete.
To this reinforced concrete central structure is added a high-tech glass and steel cladding costing an estimated 100 million dollars.
The engineers of the Burj Khalifa had to find a solution to prevent the burning desert sun transforming this magnificent glass skyscraper into a gigantic oven.
In 1947, the UN wanted to cover the United Nations Building (154m) in New York entirely with glassbut had to find a way of cooling the building to avoid transforming its 39 floors into a veritable greenhouse.
The glass walls not only let in more light but also more solar radiation, generating large amounts of heat that needs to be evacuated.
The solution would come from an American engineer called Willis Carrier (1876-1950) who designed and manufactured the very first modern air conditioner.
Thanks to air conditioning, the construction of skyscrapers such as the Burj Khalifa would be possible even in places with a very hot climate.
With temperatures sometimes exceeding 40 degrees Celsius in the shade, the Burj Khalifa cannot use conventional glass. Glass with a specially designed double-sided coating is used to reflect both ultra-violet and infra-red rays. The Burj Khalifa has more than 30,000 glass panels with a surface are of 103,000m² (equivalent to 12 football pitches) and uses more than 1 million litres of water a day to cool it down.
Stage 4: Speed of construction
Before New York’s Twin Towers (417m) became the highest structures in the world in 1970, their developers were looking for ways to reduce the construction time to a strict minimum because of the elevated projected daily costs of construction.
Part of the solution would be to pre-manufacture elements of the tower off-site before transporting them and assembling them on-site like a giant puzzle. But how to go about lifting and assembling these parts sometimes weighing up to 50 tons?
The engineers would travel to Australia to source a crane (Favco Standard 2700 Crane) capable of lifting up to 50 tons, and just as importantly, to the vertiginous heights necessary for construction of the Twin Towers! Thanks to the prefabricated sections and these so called “kangaroo cranes”, the workers were able to build at a rate of nearly two floors a week.
However, during the construction of the Burj Khalifa, the key to speed was a different technology called “Jump Form”.
The sections of Burj Khalifa are built on the spot, one section after the other, gradually climbing in height!
One problem with this system, however, is that the higher the building gets, the more difficult it becomes to pump the liquid concrete up to the construction level at the top.
In order to cope with the weight of up to 25 tons of concrete inside the piping, immensely powerful pumps (630 HP) had to be used! Even so, it would take almost 40 minutes for the liquid concrete to travel up the pipes from the ground to the 155th floor. The system had to be ingeniously designed not only to meet the power requirements of pumping but also had to take into account the chemical properties of the liquid concrete. If the concrete were too liquid, it would take longer to harden with the inevitable ensuing delays. If on the other hand, it was too thick, it risked solidifying too quickly and blocking the pipes.
This project costing close to 1.5 billion dollars left very little room for error. In the end, it all worked perfectly though and a new floor was added every 3 days!
Prefabrication allows for the construction of giant skyscrapers in record time, making them more profitable!
Stage 5: Wind!
Built in 1970, the 100-story-high Sears Tower (442 meters) is regularly subjected to tremendous bursts of wind reaching up to 80km/h coming off Lake Michigan. This made the use of a traditional steel skeleton impossible, which would have permitted the building to sway so much as to make its occupants queasy.
In order to combat this problem, the engineers decided to take the design of traditional structures and turn it inside-out. The resulting “exoskeleton” would now give the building the necessary rigidity to handle the high wind speeds.
The Burj Khalifa, however, is two times taller than the Willis Tower in Chicago! At such a height, a simple exoskeleton is not rigid enough to combat the forces of the wind. Strong bursts of wind pose a serious threat to skyscrapers. The air flowing around the building can form into circulating masses of air called vortices, a bit like mini-tornadoes. These create areas of low pressure that can have a vacuum affect pulling the building towards that vacuum. The higher the building, the more powerful and dangerous the vortices generated.
Rather than trying to battle the wind conventionally, the team of designers of the Burj Khalifa decided to combat this phenomenon using aerodynamics and by giving the tower the quite the unusual shape we see today. The building rises into the sky in several different sections each topping out at different heights around the central core. This unusual design deflects the wind around the structure and inhibits the creation of the aforementioned vortices or whirlpools.
Stage 6: the foundations
What special measures did engineers have to take to be able to erect such a tall building on the sandy surface of the desert?
To support the extreme weight of the Burj Khalifa’s more than half a million tons, it was necessary to dig at least 50 meters underground to find rock, but it was fragile and covered in groundwater, and digging large holes in it could cause its collapse.
The use of a viscous liquid called polymer made it possible to form the 200 concrete foundation pillars, essential to support its incredible weight and prevent it from sinking into the sand.
It took just over 130 years for the power of human ingenuity to overcome all of the technical challenges that the construction of these skyscrapers posed. But with ever higher buildings, vulnerability increases.
Stage 7: Evacuation
To protect the 35,000 occupants of the Burj Khalifa in case of danger, 9 shelters built with reinforced concrete walls and protected with fireproof sheet metal are located every 30 floors, providing 2 hours of protection in case of fire. These shelters are provided with a special air supply system and sealed fire doors to prevent smoke infiltration. A warning system operates around the clock, and takes action at the slightest detection of smoke.
The Burj Khalifa, the king of the skyscrapers, is the highest structure ever built by mankind, and will probably remain so. (At least until someone builds a bigger one!)