Did you know the world’s tallest building, the Burj Khalifa, and mobile phone company Samsung have a connection? Skyscrapers such as the Burj have come a long way from The Home Insurance Building, constructed in 1885 and commonly referred as the first skyscraper, which was merely 138 feet in height and had ten stories. Today, Burj Khalifa is the tallest freestanding structure made by man with a staggering height of 828 meters. If the name Burj Khalifa sounds unfamiliar, that’s because Burj Dubai at the last moment was renamed to Burj Khalifa in the honor of the ruling president of Abu Dhabi, Sheikh Khalifa bin Zayed al Nahyan who provided a $10 billion bailout for the financially troubled Dubai government.
There’s no exact definition to what qualifies as a skyscraper and is more like a variable with time, and geographic region as its dependent parameter. Currently, in the US as a matter of convention, buildings greater than 150 meters (approximately 45 stories) are classified as skyscrapers. As you’d know, a building in India with a height of over 100 meters would be clearly distinguishable from its surroundings and would earn the title of being a skyscraper.
The ability of living and working 100 stories above the ground present its own set of unique challenges ranging in domain from construction to plumbing. Here’s a look at some of the problems faced by designers and civil engineers while constructing a skyscraper and their solutions.
Given their height, skyscrapers either sink in to the ground, or collapse due to instability, and hence they are built around a central frame generally made of steel. Think of the steel frame as a three dimensional lattice of interconnected steel bars on which the weight of the whole building rests. This eliminates the need of the load bearing shear walls, and instead can be replaced by curtain wall generally made of glass giving massive weight advantages. This is the main reason to why skyscrapers have glass walls.
Most modern (post 1970) skyscrapers use the “Tube Frame” which was introduced by Fazlur Rahman Khan. This was done to resist lateral forces of wind and storm. Here, the whole building acts like one hollow beam positioned perpendicular to the ground surface. The exterior of the building is reinforced with intersecting spandrel beams.
A major challenge in building a super-heavy skyscraper is creating a stable foundation for the building. For example, consider the Burj Al Arab, famously known as the yacht shaped hotel in Dubai. It took close to three years to reclaim the land from the sea and form the foundation. The rest of the building was completed in approximately the same time frame. To maintain stability, 230 concrete pipes (each 40-metre long) were drilled in to the ground.
Similarly, the Taipei 101 tower built in earthquake prone Taiwan has 382 of such concrete pipes (each of which is 30 meters in height and 1.5 meters in diameter) drilled into the ground to secure the building from earthquake. Approximately 15 months were devoted to the construction of this foundation. It was the tallest building at the time of its construction.
In the case of Burj Khalifa, there were 192 pipes with lengths exceeding 50 meters each. Reinforced concrete forms the main structural system for the building, a total of 3,33,000 cubic meters of concrete is used for the building, by comparison a typical water tank in a household is of 1 cubic meter capacity. Constructing this massive building took a total of 22 million man-hours.
The climate of the construction site provided added challenges due to the heat in the daytime with temperatures exceeding 50 degree Celsius. Therefore, the concrete was poured during the night (the air is much cooler and the humidity is higher, which is essential for curing the mixture evenly) mixed with ice and was pumped to a staggering height of 601 meters creating a world record of its own. Previous record for concrete pumping was held of the Riva del Garda Hydroelectric Power Plant in Italy in 1994, when concrete was pumped to a height of 532 meters. At the peak of its construction, the Burj Khalifa was adding a story to the structure every three days and hence it was absolutely essential for the concrete mix to not form any cracks and cause any delays.
Constructing structures so far above the ground faces serious wind problem. To give an example, the wind velocity at the top of the Burj Khalifa can be thrice the velocity at the base. Given the climate of Dubai, wind speeds at the base itself can exceed 50 kilometers per hour. The Chicago- based architectural firm Skidmore, Owings & Merrill, who were responsible for the design of Burj Khalifa were aware of this challenge and to counter it they had contracted a Canadian-based wind engineering firm RWDI, who was responsible for checking if the various designs of the building made by the architects were aerodynamically efficient. According to Wayne Boulton, GM (Wind Energy) at RWDI, “We constructed a scale model and put it in a wind tunnel. In the wind tunnel, we are able to test a number of different wind speeds and directions. We can test the pressure you would get on the surface of the building under normal conditions and also under more extreme events.”
Extreme engineering solutions like Wind Tunnel simulations, generally reserved for race cars, are used in the construction of Modern Day Skyscrapers
With more than 10,000 people living in a confined space of a modern day skyscraper safety becomes an important issue. There are two aspects for it – one is protecting from man-made mishaps such as fi re and electricity, which is relatively easier to tackle. The other is protecting a structure more than 500 meter above the ground from forces of nature such as typhoons and earthquakes. We shall look at two counter systems to get a feel of structural safety measures in skyscrapers.
The Taipei 101 Tower sits merely 660 feet from a major fault line in Taiwan, or in simple terms is very prone to earthquake damage. To counter this, the construction engineer first wrapped the entire building in a massive steel cage of immensely welded steel columns that is designed to flex and bend to absorb seismic energy. The engineers proclaim that this can withstand the impact of the most powerful earthquake in 2,500 years.
A second counter measure is the inclusion of a Tuned Mass Damper (TMD), which also prevents damage from lateral forces such as strong wind. This innocent sounding device is actually a 730 ton ball suspended by eight solid steel cables. This device acts like a giant pendulum and counters the motion of the building due to seismic and atmospheric forces. It was designed and constructed by firm called Motioneering. It can move to a maximum of five feet in any direction and is surrounded by a protective bumper ring to prevent it from swaying too far.
Another example of the installation of a Tuned Mass Damper is an oil based hydraulic system in the Citicorp Center in New York. The hydraulic system adjusts the position of the a 400-ton concrete weight block located on one of the top floors and in effect adjusting the entire weight of the building from one side to another. A powerful array of computers and sensors monitor the wind forces around the building and generate the stimulus for the movement of the block.
Burj Khalifa also has the provision of pressurized, fire-proof, structurally reinforced and critically air-conditioned refuge chambers located after every 25 floors, so in case of any disaster, natural or manmade disaster occupants needn’t climb down more than 160 floors to find safety.
Another unique challenge is the skyscraper causing earthquakes in the surrounding region. Many had believed that the Taipei 101 tower had reactivated a dormant fault line. In order to counter this argument the designers and construction contractor of the tower had ensured that they remove equivalent amount of soil from the foundation as the weight of the whole tower, a figure close to a staggering 7,00,000 tons in this case.
It’s not rare to see lots of people standing next to windows or in the open lobby in multi-storied building hooked to their mobile phones because of poor signal strength in their offices. Imagine this problem at the scale of Burj Khalifa. The engineers at task have handled this problem in style. Two local companies Du and Etisalat have provided complete connectivity inside the building including elevators and lobbies at speeds exceeding anywhere else in the region. The building is equipped with state-of-the-art HSPA+ 3.5G mobile connectivity that offers speeds up to 21 Mbps for download and 11 Mbps for uploads.
One of the primary things that make a skyscraper practical is the inclusion of elevators. Choosing the number, size and configuration of elevators is a major decision because increasing the number of floors in a building leads to more occupants who need more elevators to carry them, but elevators occupy a lot of valuable floor space and hence decreasing occupancy potential.
The Burj Khalifa, not surprisingly, has the fastest elevator in the world and can travel at a peak speed of 64 kmph – faster than some of the first aircraft. At this speed, you can descend more than 4 stories in a single second. Even the relatively slow elevators in Taipei 101 can transfer you from fifth to 89th floor in just 37 seconds.
Most skyscrapers in order to save space and increase capacity use double deck elevators where via the same elevator shaft two floors are serviced at the same time. Obvious disadvantages include increased travel time as passengers on both floors have to stop even if the elevator needs to stop at only one floor. Burj Khalifa is also one of the few buildings in the world in which certain elevators can be used for speedy evacuation in case of fi re as opposed to conventional scenarios where elevator operation is stopped during fi re. The building service/ fireman’s elevator will have a capacity of 5,500 kg; compare this to the elevator in your building which generally has a capacity of 800 kg or less.
Simple and trivial as it may look, maintenance work post-construction in itself is a major challenge. Take the example of cleaning the windows in Burj Khalifa. To accomplish this seemingly simple task, a system worth more than $8 million (approximately Rs. 40 crore) is installed in Burj Khalifa by an Australian company called Cox Gomyl. The machinery was manufactured in Airport West and is operated in manned mode for the lower stories and completely unmanned after that. It takes about 3 months to complete the whole building, so yes, as you can imagine, it is dirty by the end of a cycle and the system is operated continuously.
Horizontal tracks are fixed on the exterior via which small transport hubs move carrying workers and equipment, when not in use they are tucked away out of sight to maintain aesthetics. The tracks are located on the level number 40, 73 and 1,009. Each track is equipped with a 1.5 ton bucket machine which can slide over the tracks horizontally and then will descend vertically using strong cables to cover the whole building.
A seemingly unimportant consideration on the surface, but having the tallest building in your country has become an issue of national pride. It is similar to hosting the Olympics or Commonwealth games and exhibits the technological and economic advancements of your nation. The task of ensuring that such a symbol of national pride is aesthetically appealing is taken very seriously by the designers and can cost a major fraction of the total time and money spent on the project.
For example, there are over 1,000 pieces of art that will be on display inside the Burj Khalifa. The lobby alone boasts of 196 cymbals constructed out of bronze and brass alloy, each representing one nation of the world. They are plated by 18-carat gold and individually produce a distinctive sound when struck by dripping water, intended to mimic the sound of water falling on leaves.
On the outside stands an electronically controlled fountain designed by WET Design (the California-based company whose other installations include fountains at the Bellagio Hotel Lake in Las Vegas) at a cost of $217 million (over Rs. 1,000 crore). The fountain is fitted with over 6,000 lights, each of which can be controlled electronically and has 50 colored projectors. The 275-metre long fountain can shoot water up to a height of 150 meters (that is over 40 stories, so yes if you are reading this article anywhere in India, the fountain can drench you on your rooftop) is a technological marvel in itself.
The race to build the tallest and strongest includes the use of wind tunnels, material research which ironically involves use of Nano particles to form stable mega structures, sophisticated digital monitoring systems, and automated cranes and so on. As cities get ever more crowded the next natural step is to construct building over 1 kilometer in height and you might not have to hold your breath far too long for it since the Saudi Prince Al Waleed bin Talal is already on the job.