Going up: Vertical transportation in the 21st century
Both conventional wisdom and Malthusian theory say that the earth is presently overpopulated with more than 6.5 billion people and that we are rapidly running out of land on which to build new cities or expand existing ones. The reality is far different. As demographers will tell you, the entire population of the world could fit in the state of Texas—and there would still be more square feet per person than now exists in Hong Kong or Singapore!
What is true is that the world is in the midst of an immense population shift from rural areas to superbig, superdense cities. This mass urbanization is intensifying demand for office, retail, and apartment spaces. In the world’s most expensive high-rise cities, such as Hong Kong, London, Tokyo, and New York, the free market axiom that real estate should be developed for its “highest and best use” has never been truer. Generally, the more expensive the land, the taller the buildings and the higher the rent. In Hong Kong, for example, it often makes complete economic sense to tear down buildings after, say, 20 to 25 years and replace them with taller, more valuable towers. Along the Chinese Pearl River Delta, spreading from Hong Kong to Shenzhen and then on to Guangzhou, 100-story-plus mixed-use towers are becoming routine.
From the 1950s through the late 1980s most of the world’s supertall towers—those greater than 75 stories—were constructed in North America, mostly as office towers. Starting with the twin, 88-story Petronas Towers, in Kuala Lumpur—which were finished in 1998—construction of the world’s tallest towers shifted to Malaysia, then to Taiwan with the Taipei 101 office tower, and now to Dubai with the 163-story, 2,717-foot-high Burj Khalifa, which consists mostly of residential and hotel space.
But trees can’t grow to the sky and neither can buildings. Or can they? In the city of the future, the most important transportation system will not be cars, buses, subways, or even futuristic monorails. It will be the elevator. In a densely populated high-rise city, vertical transportation becomes far more important than horizontal systems. Think of it this way: while you wouldn’t hesitate to walk a block or two on a level sidewalk to an important business meeting, you’d be much less enthusiastic about traveling that far by stair.
No matter how tall the building, all floors need to be easily accessible. A supertall office tower could have tens of thousands of daytime occupants and if it’s well designed, none of them should wait longer than 20 to 25 seconds to begin their elevator journey. The more people and the more floors, the greater the number of high-speed elevators required.
But more elevators means that more valuable floor space will be lost to shafts and equipment. That reality helps explain why so many of the new super-tall buildings springing up around the globe sport a different shape. It used to be that an individual tower’s economics would dictate its function—whether office, hotel, residential, or mixed use—as well as its height. Now, it is quite common to design supertall, iconic towers for reasons other than pure economics; perhaps to place a country or city on the world scene and to make the surrounding real estate plots more valuable. At the same time, it’s becoming harder to justify building a supertall all-office tower in the 3-million- to 5-million-square-foot range. Such a building would have a projected tenant population of 15,000 to 25,000 people and would have a profound impact on the surrounding surface transportation. It would also require huge floor plates, immense structural framing, and, of course, more elevator units along with their vertical shafts. With those numbers, a building could lose 40 percent of its interior space to elevators. So when reaching for the world’s height record these days, it’s easier and more efficient to design supertall structures that are needle-shaped, with upper floor step-backs and thus smaller floor plates, much like New York’s Empire State Building. For example, the world’s second tallest building, Taipei 101, has only 2.3 million square feet of office space compared with 3.8 million for each of the former 110-story World Trade Center towers in New York.
In addition, these new supertall showpieces tend to be designed for mixed use. The various usage zones are usually “stacked,” with the retail or podium levels on the bottom floors followed by office zones, hotel floors, residential levels, and, at the top, observation and restaurant levels. Each of these sections would normally have its own set or zone of express and local passenger-elevator groups. In contrast to the office tower designs, such a building would contain only about 4,000 people.
That makes a big difference when it comes to designing the elevator systems. Typical tower-elevator speeds range from 500 feet per minute to 2,000 feet per minute. The platform capacity is normally in the 3,000- to 4,000-pound range and nominal car usage runs between 16 to 22 people per trip. In office-zone designs, elevator groups are typically arranged in six- to eight-car single-deck or double-deck groups, with each group, or zone, serving 15 to 18 floors. That means a 15-story office tower is normally equipped with a single, low-rise elevator zone, a 30-story building would have two zones, a 45-story, three zones, and a 60-story, four zones. Beyond 60 stories, a fifth local elevator group is seldom utilized because this arrangement would consume too much valuable tower core space. Above that height, buildings usually have elevator shuttles that provide express service between the ground floor and a sky lobby, located at, say, level 61. Passengers transfer there to another set of four local elevator zones, to reach the upper stories.
The beauty of the sky-lobby paradigm is that it permits the upper-floor local lifts to be stacked on top of the lower local-lift zones without consuming additional core space. In a multiuse tower, the upper hotel and residential zones are normally provided with their own sets of sky-lobby shuttles. These other usage zones have much different elevator requirements from office zones. A single elevator bank can typically serve 25 to 30 floors in a hotel zone and about 35 to 45 floors in an apartment environment.
The New York World Trade towers, built in the 1960s, were each 110 stories tall. The Burj Khalifa, which opened in January of 2010, is 163 stories tall. We are presently working on the elevator designs of the next world’s tallest supertower, to be located in Jeddah, Saudi Arabia, which will have 250 stories and be 1 kilometer (3,300 feet) tall. It’s impossible to say how long this building will hold the crown as the world’s tallest. We can say, however, that cities will continue to get bigger by growing taller, and it will be the elevator that makes this vertical expansion possible.