Hong Kong at Night 
Skyscrapers are of essential importance to urban lifestyles. They can provide a large area for living and working in cities where land costs are often prohibitive, or where land is simply unavailable, while at the same time serve as symbols of national pride and wealth. To build even taller skyscrapers would allow for lower property prices and improved comforts for people, such as more space. The construction methods to stabilize the world’s tallest building have been considered, as well as how the potential to fail in rare events such as fires, decommissioning, and terror attacks are much higher, as there would be great complications in quickly escaping, and large debris can damage the area.
It has been determined that the tallest skyscrapers use novel solutions, such as setbacks and a Y-shaped cross-section instead of a Tuned Mass Damper (TMD), to solve the problems of increased wind flow and torsion over the building. Further, it has been shown that the potential for failure is great in these tall skyscrapers, and although the lifetime might not be certain, many redundancies can be added to ensure its potential to failure is at a minimum.
As the population of large cities increases and the available land area stays fixed, there will be a more pressing need to house businesses and people at a non-prohibitive price, and Megatall Skyscrapers are the solution.
The first-ever skyscraper was the Home Insurance Building in Chicago, USA. When construction finished in 1885, it had a height of 42 meters; shorter than the Great Pyramids. By modern definition, it would just be a building. However, unlike a normal masonry building, it had an internal frame that supported the building; a feature that is characteristic only to skyscrapers. It was also fire-resistant, light, and supported by steel. All these characteristics are common only to skyscrapers.
From then on the race to make skyscrapers started. They became increasingly a show of both national and corporate power and wealth. More than that though, they served an essential purpose. Managing to keep large cities relatively small, with an ever-increasing population and with more and more companies trying to establish a presence in large cities. The Empire State Building, which held the title of tallest building for 41 years, covers a site of 7366m2, but the area of the floors inside the building accumulates to 208,879m2. That is an increase of about 2800%. Even though not all of this area is usable (there are usually support structures inside the building), the increase is still significant. 
In areas where land is expensive or simply unavailable, like the island of Hong Kong, then this large increase is often desirable. Whether or not a megatall skyscraper (>600m), would be more efficient than several small skyscrapers is questionable, but as available land decreases in large cities and populations increase, one day they might become necessary, or at the very least useful. To drive down the costs, several challenges must be resolved beforehand.
There is a vast array of structures for existing skyscrapers, each trying to overcome gravity and wind. For skyscrapers, wind brings several complications. Other than serving as a perpendicular force trying to turn it and push it, it also causes it to vibrate. Since air cannot ‘bend’ around corners easily for blunt objects, then that means the pressure is lower on the edge opposite to the direction of flow of air, and a ‘vortex’ will split off (See Figure 1). The building will move to that low pressure area, which will form on alternating sides of the building. This effect is known as vortex shedding. When the vortex has the resonant (specific) frequency of the structure, then that will cause the entire building to vibrate. If it is too much or over a long timescale, then the entire skyscraper will collapse. It can also be catastrophic if the direction of the wind and of the sway are synchronized, this results in constructive interference, meaning more oscillations, leading to a great deal more damage. The friction between the air and the surface and the type of material can usually help dissipate some energy. However, when the building becomes megatall, then the surface area increases leading to potentially more sway.
Figure 1: A diagram to represent vortex street caused by multiple vortices. Courtesy Cesareo de La Rosa Siqueira.
The fundamental frequency has this equation:
Where l is length, T is tension and is mass per unit length. Tension will be dependent on the material and gravity, and the exact length will depend on the needs that the building has to fulfill. The mass per unit length can be problematic. As materials get lighter, more flexible, and stronger, and there are less non-structural components to buildings, like bulky internal walls, then modern buildings have a smaller mass per unit length, leading to a smaller fundamental frequency. The lack of old-style concrete walls inside the building also limits dampening from wind. Buildings that have few or no aerodynamic properties, like the Trump World Tower (Figure 2), create a lot more vortices and thus need some way to prevent swaying. 
Figure 2: Trump World Tower in New York. It is an example of a tall skyscraper without any major aerodynamic properties. 
There are various methods that can be used in combinations to solve this. One solution is to use a tuned mass damper (TMD). This is a pendulum, usually residing on the top floors of a skyscraper. Its frequency is set to that of the structure, via a computer. When the building sways, the TMD is set to move in the opposite direction. This effectively puts the building and the TMD out of phase (See Figure 3). A hydraulic press will transfer the inertia from the TMD to the building, thus attaining rest. 
Figure 3 – A TMD in the Taipei 101 building in Taiwan and two waves out of phase.  
The building is continuously accelerating backwards and forwards when swaying, so a force in the opposite direction to that in which it is accelerating will effectively create a resultant force of zero.
That in itself is not enough for megatall skyscrapers. There must be other factors which help in stabilizing the structure. The Shanghai Tower, the world’s second tallest skyscraper with the world’s heaviest TMD (907 metric tonnes), has magnetic stabilizers. When a conductor passes nearby a magnetic field, some of its kinetic energy is converted to thermal energy while the magnetic field changes polarity, thus being dissipated to the surroundings and lowering the building’s kinetic energy.  
The Burj Khalifa is the world’s tallest building, and it does not have a TMD. Therefore an alternative is to look at the physical structure of the building and how aerodynamic it is.
Figure 4 – Burj Khalifa (Previously known as the Burj Dubai), the world’s tallest building. At the bottom are the cross sections at different heights.  
Geometrically speaking, the most stable design is that of a pyramid, hence one of the reasons why the Great Pyramids still exist. By having less and less mass as you go up allows for the minimum possible force exerted on the bottom. However, such designs (called setbacks) reduce the available floor space for upper levels, making the design less efficient.
Nonetheless, these setbacks provide increased stability with a small decrease in usable space. The increase in stability is provided by lowering the centre of mass of the building, making it lighter and decreasing wind exposure. This is used in a unique way by the Burj Khalifa. Since the setbacks are not applied symmetrically, this disrupts wind flow, reducing vortices and thus increasing stability.  Even the orientation of both the setbacks and the building is relative to the frequent wind directions of the city, which in itself shows the hidden factors behind every decision. 
The other structural element is the building’s Y-shape. It increases the surface area available for each level, therefore increasing natural light and views from the hotel rooms.  It greatly reduces torsion on the building, by blocking the wind’s ability to rotate around the building (See Figure 4) .
Figure 4 An example of torsion on a building. 
A popular type of concrete, Portland cement concrete, has a maximum average density of 2.4 tonnes per meter cubed.  One wonders how 330,000 cubic meters of concrete were used in the Burj Khalifa’s construction, and why such a bulky material is preferred in almost all tall skyscrapers, as well as why new technologies were developed to pump concrete to a record 606m.    The Burj Khalifa has a tube structure, like many supertall (300m<length<600m) skyscrapers such as the Willis Tower in Chicago, (previously known as the Sears Tower). That is a design in which a tube at the centre of the building, in combination with exterior walls connected normally to it, can take on the external forces. This means that there is no need for interior columns, meaning there is more available space and is able to overcome moments by spreading them more evenly. A bundled tube system, which both the Willis Tower and Burj Khalifa have, is where there are multiple self-supporting tube structures which can strengthen each other.   The core of the structure is thus usually made of concrete for cost, ease of work, and physical properties, being a perfect balance of the three. 
Assessing this information, it is easy to see the wide range of concepts that have to be considered when trying to make a megatall skyscraper. Tube construction helped with structural integrity, aerodynamics (having the benefits of removing the TMD, which has maintenance costs) and increasing available floor space. Setbacks reduce the centre of mass and prevent the formation of vortices. Using a Y-shaped cross-section increases available floor area, natural light and serves to breakup wind currents. Small decisions which normally wouldn’t have to be considered for standard skyscrapers can have massive repercussions here, and hence the need for extensive multidisciplinary collaboration and careful planning.
Unfortunately, 30% of the Burj Khalifa’s height is due to its spire. Without it, it couldn’t be classed as megatall. In fact, none of the world’s 3 megatall buildings would be ‘megatall’ at all. Taking out of consideration spires, the ‘Burj Khalifa’, ‘Ping An Finance Center’, and ‘Shanghai Tower’ would all range 560-580m. Therefore technology is close in creating relatively affordable megatall skyscrapers if building techniques continue to progress at the current rate. Even though the Burj Khalifa is not habitable up to its 828th meter, the structure still supports the heavy spire, so the limit is definitively not in the 560-580m range. 
The potential for failure in a large building is much bigger, as shown by a simple calculation.
The Burj Khalifa has lifts capable of reaching 10ms-1. 
It has a double decker lift, capable of carrying 12-14 people per cabin. Let us assume there is a mass equivalent to 13 people per cabin. 
The approximate mass of an Otis double decker lift is 10,000kg  
Since the lift is aerodynamic, then one can assume the top speed is the same as the terminal velocity. (Terminal velocity is the maximum speed an object can reach, with aerodynamics being the limiting factor.)
Energy = 0.5 x mass x velocity2
E = (1/2)mv2
One person has a mass of about 70kg 
1 gram of TNT releases 4184J 
591000/4184 = 141.3g
So the energy released would be equivalent to 141.3g of TNT.
That is quite a lot. Considering there are 65 elevators, if they were all to fail (somehow) under the same conditions, then there would be a potential for over 9 kilograms of TNT, which is more than enough to do some very serious damage to the building.  
Of course, these calculations are very abstract and fictitious. But other events such as fires, natural disasters and terrorist attacks all have great potential for disaster, and are much more realistic. Even though very tall skyscrapers have fire lifts and evacuation plans, they are no less vulnerable to fires.
In the tragic events of 9/11, the Twin Towers, which were then the tallest skyscrapers on Earth, caused the collapse of the adjacent building due to resulting debris and air pressure. Structures now are no longer built using the same principles, to prevent the same tragedy, and security is often placed first. For example, modern building codes have additional fire exit stairways, fire service elevators, high reliability sprinklers, and emergency radio communications.   Although it is impossible to predict the likelihood of terrorism or natural disasters, buildings can be built to be redundant, which will serve to protect it from all sorts of unexpected problems; as well as extending its lifetime.
Other problems such as decommissioning come to mind. Skyscrapers usually get refitted or repurposed, but rarely decommissioned. Although there is not a consensus on the lifetime of a skyscraper, the maintenance cost is high and land rent is costly in large cities. If the skyscraper is no longer profitable, and the problem cannot be solved by selling it, it would be problematic to decommission one. If they are to become more common, then at least some buildings might face this dilemma, and new engineering techniques will have to be developed accordingly.
Perhaps these large skyscrapers will be most useful in cities like Hong Kong, which has the world’s least affordable housing prices . Having tall skyscrapers which can house thousands, and include markets, facilities and jobs will serve to reduce the housing prices in the small island, with its citizens being able to live more comfortably. Other small islands like the city state of Singapore and large cities like London can easily find a use for these buildings, especially when taking into consideration high property prices.
If there are to be ‘vertical cities’ built in the future, then more megatall skyscrapers should be built first. This would allow for a more distributed understanding in the engineering profession of how to build them, and to ensure that competitiveness creates cheaper superstructures, allowing for the derivation of realistically priced kilometre tall skyscrapers.
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About the Author
Greg is an 18 year old, passionate in all aspects of engineering. He’s studying his first year of mechanical engineering. In his spare time, he enjoys reading, playing the piano, and taking long walks.