What's the tallest skyscraper physics would allow us to build?

Credit: via Flickr

In just a few weeks, architects will find out if the tallest building in America really is 1,776 feet tall. One World Trade Center technically soars to that height, but about 408 feet of it isn’t building but “spire.” The arbiter of tall buildings, the Council on Tall Buildings and Urban Habitat, has yet to decide whether that's going to count.

If the spire is considered an antenna rather than part of the building, One World Trade Center will be the third-highest building on the continent, behind two Chicagoans: the Willis Tower and the Trump International Hotel & Tower. While that’s still impressively tall, a height of 1,368 feet (where the spire begins and occupied space ends) is decidedly less patriotic.

Regardless of what the council decides, the Willis Tower is higher in a practical sense: its highest usable floor is 83 feet taller than One World Trade Center (previously called Freedom Tower). And these structures are still modest when compared to the behemoth buildings either already open, under construction, or planned in the Middle East and Asia. But how tall could skyscrapers really get? Could we build a mile high? Or taller than Mount Everest? It turns out to be a question of semantics almost as much as one of physics.

Wind and earthquake safety

By a long shot, Earth’s tallest man-made structure is the Burj Khalifa in Dubai, climbing 2,717 feet (828 meters) into the sky. It’s 745 feet higher than the next tallest, the Makkah Royal Clock Tower Hotel in Saudi Arabia. Building the Burj required a whole new architectural design, called the buttressed core. It’s shaped like a tripod, with three main legs anchored to a central core that's responsible for most of the height. Rather than relying on super-strong steel and concrete mixtures, its very design gives it stability — each wing buttresses the other two.

Even with these architectural features, at some point, the amount of steel needed to stabilize higher floors becomes too heavy, forcing the building to become thinner. But this also protects it against wind. The Burj rises in a jagged pattern, with uneven levels topping out at varying heights around the structure to deflect gusts and prevent air vortices.

Protecting supertalls from earthquakes is another challenge. To study building behavior, architects place scale models in wind tunnels and on shake tables, measuring how concrete buckles. Buildings are designed to withstand the ground motion they’d experience in an earthquake of a given magnitude (which depends on various building codes). Earlier this year, Chinese researchers writing in the Journal of Constructional Steel Research ran computer simulations what would happen to a supertall building in an overpoweringly strong earthquake, and found it would collapse in a pancake. But the shaking required to ruin such a structure is unlikely to occur, they wrote.

The Council on Tall Buildings and Urban Habitat asked a group of star architects last fall what they considered the biggest challenges to buildings reaching a kilometer or a mile high. Most think that it will be possible to do this: "the way the mind and the imagination works, engineers and architects and clients will find ways to build taller and taller,” said Matthias Sauerbruch of Sauerbruch Hutton. Aside from financing it, the biggest challenges to supertalls are much more pedestrian, as it were.

 

Elevator height

 

Moving people is probably the biggest challenge in the construction of supertalls. The most advanced elevators available, installed in the Burj Khalifa, can only climb 504 meters, so people have to get off and board secondary elevators to continue their journey to the top. Speed is another problem — the fastest elevator in the world, inside Tapei 101 Tower in Taiwan, rockets upward at roughly 38 MPH. Elevators can only descend at two-thirds that speed. Those limits are for us, not the technology: any faster and most passengers’ ears couldn’t withstand the pressure changes. So, for a building a mile high, you can expect very long rides and lots of transfers.

Taipei 101

The higher the building, the wider the base that’s you need, so at a certain size simply getting across the ground floor poses another challenge. Commuting to work would take on a whole new meaning if you have to use a subway to get to the building, and then another subway or transportation system to move around inside it. It might make more sense to plan mega-talls as self-enclosed vertical cities, with work spaces, living spaces, grocery stores, entertainment, and other amenities together, said Werner Sobek, of Werner Sobek Engineers. “If this would be the case, if it was a vertical city, then I think the only limiting factor is transportation,” he said.

Architects could also get creative and employ Eiffel Tower-style designs with hollow bases. Buildings could be built to crouch above other, smaller structures — otherwise, the supertalls would also be superwide and probably too big to fill up.

What is height, anyway?

Like One World Trade Center, many of the world’s tallest buildings are actually shorter than they look, with the uppermost accessible floor lying some distance (and often a considerable distance) below the tip of the spire. This phenomenon, which the CTBUH calls “vanity height,” has increased roughly 400 percent since the mid-1970s.

Without vanity height, 44 (that's 61 percent) of the world’s 72 supertall buildings would measure less than 300 meters, losing their supertall status, according to the CTBUH. The tallest of these is the 390-meter CITIC Plaza in Guangzhou, China.

Buildings that reach a kilometer or higher, including the planned Kingdom Tower in Jeddah, Saudi Arabia, will likely meet that height with similar architectural features. But it’s certainly possible to go a mile up, maybe even higher than a mountain, some architects argue. At that point, the challenge becomes keeping people alive and healthy and comfortable, and designing buildings to compensate for lower atmospheric pressure and less oxygen.

Theoretically, Sauerbruch said, there’s no limit. “Whether this makes sense or not is another question,” he said.

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