The Building Technology Behind a Mile High Skyscraper
Experts are predicting that there could be at least one mile-high skyscraper by 2050. There's also likely to be a trend of buildings over 3,200 feet high. What kind of tech will it take to bring those visions to life?
The current high-flying record holder is the Burj Khalifa in Dubai, at 2,717 feet, or about half a mile. Its successor, the Kingdom Tower in Jeddah, Saudi Arabia, will be the first building to surpass 1,000 meters or 3,281 feet when construction is completed in 2020.
Experts say there will be more ultra-lofty buildings to reflect the ongoing urbanization of the world's populations. Planners are predicting that by 2050, 6 billion people will live in cities. Already, urban areas in China and the Middle East are building up rather than out to accommodate more people with less impact on the land mass.
Data from the Council on Tall Buildings and Urban Habitat indicates that several towers more than 3,720 feet high will be built, and there's about a 10 percent chance a building exceeding 5,280 feet will be built by the middle of this century, MIT Technology Review reported.
Of course, those predictions assume a relatively stable political and economic climate and a lack of a zombie apocalypse or asteroid strike. We don't want to be like the scientists at Jurassic Park, who were so consumed with how to clone dinosaurs they never stopped to ask why. The same thing goes for the mile-high skyscraper.
To prepare for the next generation of supertall buildings – those over 300 meters or 984 feet – engineers are working on innovations such as carbon fiber cables for elevators and mass dampers, weights that reduce sway from the wind. An elevator that uses magnets instead of cables could move through buildings sideways like the Wonkavator from "Willie Wonka and the Chocolate Factory."
What construction technology will be required to build a mile-high skyscraper?
Rethinking Structural Design
Reaching a mile high building isn't as easy taking a half-mile high building and doubling everything. Materials and spaces don't scale up like that. That's why tall buildings taper toward the top. Once you get past a certain point, the investment in additional steel and concrete doesn't deliver much usable space. Tapered structures also allow for light and air movement in cities, and have been a factor in building codes for over 100 years. There is a point where if the goal were to end up with the most usable interior space, it would be more efficient to construct two smaller buildings.
As building and material science advanced, tall buildings were designed on a tube system rather than a rectangular design of the earliest skyscrapers. The tube works well to a point – the width and depth must rise in proportion with the height. Pretty soon you end up with millions of square feet of mostly undesirable office space.
The Burj Khalifa used a new approach — the buttressed core. A central hexagonal concrete core is supported by three triangular buttresses, like the fins on a rocket.
Designed by Architect Adrian Smith, who also led design on the Burj Khalifa while still at Skidmore, Owings & Merrill, the tower is supported by a central core, which is braced by 2-foot-thick concrete walls that line the double-loaded corridors in each of the three wings in the tripodal structure.
William Baker, the top structural engineer at Skidmore, Owings and Merrill, who worked with Smith on the Burj Khalifa, thinks it's possible to use the buttressed core design could be used to build a tower mile-high tower, he told City Lab.
One variation under consideration would use a hollow base, like the Eiffel Tower. The bottom would be open; otherwise, the ground floors would be too big to make effective use of as the building rose to the sky.
Raising Elevator Expectations
Moving people in a supertall building is one of the biggest challenges. To reach the upper floors of a mile-high building with current technology, passengers might have to change elevators several times, because one elevator system won't work.
Right now, elevator runs are limited to about 1,600 feet because traditional wire suspension ropes can't support their own weight and the weight of the cabin beyond that length. That means building multiple elevator lobbies in supertall buildings that take up valuable floor space for mechanical systems.
Finnish elevator company Kone developed an ultra-lightweight carbon fiber cable, UltraRope, that could double the distance of an elevator's ride. It's slated to be used in the Kingdom Tower to reduce the number of elevator lobbies needed.
Out-of-the-box thinking led ThyssenKrupp to eliminate cables altogether. Each cabin in the MULTI elevator design is outfitted with a stack of permanent magnets that interact with electric coils on the hoistway. The coils pulse on and off to push the car in the right direction. Like the Wonkavator in "Willy Wonka and the Chocolate Factory” film, the MULTI elevator can move side to side as well as up and down.
Without cables confining one elevator cabin per shaft to vertical movements, multiple cabins could move through a building in a loop like a bus system. The system can be built with fewer and smaller shafts than conventional elevators, increasing a building's useable area by up to 25 percent.
Using more elevators in a building can significantly reduce passenger wait time. Researchers at Columbia University found that office workers in New York City spent 16.6 years collectively waiting for elevators, but only 5.9 years actually traveling in them. The system effectively sidesteps the limits that cable-based elevator systems place on building heights.
Another limit to cloud-busting vertical living is the changing air pressure that could wreak havoc on those with colds and sinus problems as they ascend to their homes or offices. Elevator cabins could be pressurized like airplane cabins, helping ease the transition to higher altitudes. These elevator cabins could also move faster than unpressurized versions, reducing elevator-waiting time.
Battling Building Sway
Some buildings like the 35-story Grand Hyatt San Francisco are notoriously noisy as the structure creaks from wind pressure. Movement from wind can also make occupants queasy from motion sickness.
AEC professionals calculate estimated building sway from wind as a function of height and incorporate that into the plan. For the Burj Khalifa, two to four meters of movement was designed into the structure. Buildings are usually over-designed to withstand 500- to 1,000-year weather events.
Aside from noise, the biggest problem from building sway is occupant comfort. Humans can detect slight movements that pose no threat to the building. A light breeze at ground level may feel like a hurricane 150 floors above, whipping the building enough to make people feel ill.
There are two basic approaches to battling the wind: dampen it or confuse it.
Many modern tall buildings incorporate a tuned mass dampener, a counterweight that helps balance the force of movement on the building's exterior. For example, the Taipei 101 tower in Taiwan houses a 730-spherical pendulum that sways back and forth, balancing the force of the wind from storms and typhoons.
Boston's John Hancock Building and New York City's Citigroup Center also used dampers. The size and weight of the damper are customized based on the mass and height of the building. They may swing like the orb in the Taipei 101 or slide like a car's shock absorber to dampen the movement of the structure.
The Kingdom tower design uses a continuously sloped glass façade that changes the dynamics at every floor and shrinks each successive floor plate 4 to 8 inches, Architect Magazine reported. It's built on a three-legged base to imbue stability, along with other features, so a damper system is not required.
The other approach is not to fight the wind but confuse or redirect it. Buildings incorporate aerodynamic features to ensure the wind can build up dangerous levels of pressure. The main enemy is vortex shedding when wind passes the sharp edges of buildings. This condition creates eddies of air that batter the structure in unpredictable ways. Air currents can tune into the building's resonant frequency leading to vibrations or cyclic swaying that can worsen until the structure collapses. Architects are learning to direct the wind through and around the building use channels, fins and soft curves. Slavish devotion to symmetry could lead to catastrophic consequences.
The proposed 80-story, 1,2730-foot Dynamic Tower in Dubai takes confusing the wind one step further. Designers plan to have each floor rotate at the occupant's command or in unison with other floors. The tower's changing shape could reduce wind pressure as well as provide stunning views of the desert landscape.
Hacking Modern Materials
Raising a supertall building will require rethinking materials. High-tech concrete reinforced with microfibers in a complex recipe of super materials could rival the compressive strength of structural steel. Because it's more massive, a concrete tower can have the same resistance to the wind in a thinner profile. Engineers are looking at aerospace materials like carbon fiber. It's very light, very strong, and also very expensive.
The Rock Factor
Is it hubris that drives humanity to build to the skies? Hubris always ends in a hard-won lesson, and in a mile-high building that comeuppance could be massive.
Making another disaster movie connection, the 2018 film "Skyscraper" made some admittedly over-the-top speculations about what could go wrong in a supertall building. In the film, a fictional mega tall tower in Hong Kong is attacked and is engulfed in explosions and flames, and a character played by Dwayne "The Rock" Johnson must perform heroic rescues thousands of feet above the ground. While it is a work of fiction, the film highlights that ensuring the safety of people a mile above the ground would also require new approaches to fire monitoring and mitigation and evacuation procedures.
After all, you won't be able to count on The Rock to come to your rescue.