For more than a decade, the king of the skyscrapers—the tallest building in the world—has been the Burj Khalifa in Dubai. With a total height of 2,722 feet, it’s the undisputed champion of the vertical world, a megatall building constructed with a core of reinforced concrete that sits on a piled raft foundation

Since its completion, the 163-story building has become a shining part of the world’s architectural and cultural landscape, providing a soaring platform for content that will make your stomach clench. A woman donned flight attendant garb and stood at its dizzying pinnacle not once but twice to hawk for Emirates airlines, with the second stunt involving an enormous A380 aircraft flying behind her. And Tom Cruise famously scaled its glass exterior in a Mission Impossible film.

The Burj Khalifa has owned the superlative designation of tallest building in the world since 2010, towering over everything else. “That’s pretty good staying power considering that there was actually a pretty high rate of replacement—between the replacement of the Sears Tower by Kuala Lumpur’s Petronas Towers, then Taipei 101, and then we moved onto the Burj, which is considerably higher than its predecessors by a good margin,” says Daniel Safarik, the director for research and thought leadership at Council on Tall Buildings and Urban Habitat (CTBUH) in Chicago.

“That begs the inevitable question then: What’s going to be the next new tallest building in the world? And I think the answer is, we don’t know,” he adds. “Initially it was projected to be the Jeddah Tower in Saudi Arabia, but that building has stopped construction with no specified resumption date.” 

The tallest building in the world rises into the unknown 

Adrian Smith is the architect behind the Burj Khalifa, as well as for the on-pause Jeddah Tower. In a video chat from Chicago, he reflects on the question of when and if another building will surpass the height of the Burj. “I think inevitably, that’s the case,” he says.

“One of the interesting things about the ‘tallest building in the world’ as a title, is that if one is serious about doing the tallest building in the world, there’s an enormous amount of publicity that goes along with that,” he adds. Smith is now at Adrian Smith + Gordon Gill Architecture and formerly was at Skidmore, Owings & Merrill, which is known as SOM. “We’ve had clients hire us to do world’s tallest buildings before—they get an enormous amount of publicity and then for whatever reason, it doesn’t happen. Usually, 90 percent of the time, that reason is money.” 

As for the on-pause Jeddah Tower, which used to be called Kingdom Tower, Smith says that “it’s pursuing the process of starting up again,” and adds, “I have nothing that I can really disclose at all.”

Earlier this year, the Los Angeles Times took a close look at the Jeddah Tower’s frozen progress, and other mega projects in Saudi Arabia, reporting that the tower, at 826 feet tall, “remains a construction site with no construction.” 

[Related: 6 architectural facts about history’s tallest buildings]

But regardless of the Jeddah Tower’s question mark, the Burj remains a decisive and enormous exclamation point. Each time a new tallest building in the world rises up, its designers, engineers, and contractors are pushing into unexplored territory. “First of all, the structure is the most important single thing in a supertall building,” Smith reflects. “And the reason it’s the most important thing is that very few of them are done, and the history of the design process for a supertall—especially a world’s tallest—if it’s truly a world’s tallest, it’s never been done, you don’t know what you’re going to run into.”

The world’s tallest tower came from ‘a tube’

The Burj Khalifa’s core, which is supported by buttresses, is made of reinforced concrete. That’s a change from some of the classic skyscrapers of the previous century that may come to mind. “The structure of Sears Tower is all steel,” Smith says. So too is the structure of the Empire State Building, now just the 51st tallest building in the world but standing proudly since 1931. 

“The structure of Burj Khalifa is all concrete,” he adds. “And the structure for Kingdom Tower will be all concrete as well—but when I say all concrete, they’re heavily reinforced concrete structures. A lot of steel goes into that concrete.” 

Indeed, concrete technology has evolved over the decades, allowing it to have higher and higher compressive strength—the strength it can withstand as gravity pulls on it downwards. 

Stefan Al, an architect, author of the book Supertall, and an assistant professor at Virginia Tech, charts just how much concrete has improved. In the 1950s, he says, concrete was rated at around 20 megapascals. The concrete in the Burj was 80 megapascals, and today’s can do about 250 megapascals. “So basically it’s gotten 10 times stronger—or 10 times more able to withstand compression, meaning you can have 10 times more weight coming from top,” he says. “That’s certainly super impressive.” 

There’s another benefit to concrete (don’t get it confused with cement), which is the way it gets up to where workers need it—by being pumped up and then flowing out of a tube. Reinforced concrete’s current popularity is “a function of concrete’s ability to pump, because that makes it much easier to work with,” Al says. 

That’s different from working with steel way up high, because for that, Al says, “you need super-large cranes” to hoist the beams upwards. And concrete is quick. Al notes that using concrete in a city like New York can result in a building story going up every two to three days. 

Of course, pumping concrete up against gravity produces its own challenges—and opportunities to celebrate. A company that makes concrete pumps, Putzmeister, boasted that its equipment was able to get the material up 1,988 feet—a record at the time. In 2019, they looked back on that 2008 accomplishment, punning that in helping build the Burj, “Putzmeister was a concrete part.”

Smith points out that the plans for the Jeddah Tower call for it to be made out of concrete as well, including even its top spire, which on the Burj is made from steel. “Every few years, technology advances and changes—the concrete gets stronger. There are new additives, new ways of making concrete that’s better for this kind of application,” he says. “If you think about Burj Khalifa and Kingdom [Jeddah] Tower, they’re ultimately built out of a tube that’s maybe 8 inches to a foot in diameter.” He chuckled. 

The second-tallest building in the world

Words like supertall and megatall may sound vague, but in fact they have specific definitions. A supertall building is at least 984 feet tall, while a megatall stands at least 1,968 feet high. At 1,776 feet tall, One World Trade in New York City is a supertall building, but not a megatall one, and is the sixth-tallest building globally. And a new second-tallest building in the world is set to be finished this year—it’s the angular Merdeka 118 in Kuala Lumpur, Malaysia, and measures a megatall 2,233 feet tall at the tippy top. (The current second tallest building in the world is the 2,073-foot Shanghai Tower.)

But architecture is about more than height, and Stefan Al highlights an exciting diversity of design he sees in new modern buildings. “You can really speak of a new generation of skyscrapers, which are much taller, but also, you could say, more exuberant” compared to what came before, he observes. “Most of the 20th century, we only had a handful of supertall buildings, including the Chrysler Building and the Empire State Building, but now we have more than 100, and most of them have been built in the last 20 years.” 

As for buildings with wild and varied new styles, he cites the “super slender” trend in New York City, with the skinny and supertall 111 West 57th Street as a notable example. Another is the Central Park Tower, which Adrian Smith + Gordon Gill designed. 

But buildings get even more interesting. The Capital Gate Tower in Abu Dhabi may only be 540 feet tall, but it looks like it could tip over. “It deliberately leans 18 degrees,” Al points out. He says that buildings like this one “are not very logical from a structural perspective.” 

Or check out the M.C. Escher-like CCTV Headquarters in Beijing, or Mexico City’s cool Torre Reforma

So will a building ever exceed the height of the Burj Khalifa? Al thinks so, saying he anticipates it happening “within our lifetime.” 

Safarik, of the CTBUH in Chicago, is more cautious, noting that the future seems murky when it comes to a building rising higher than the Burj. But one thing is clear: When it comes to the tallest buildings in the word, things have changed since the CTBUH was founded the same year that the US landed on the moon. 

“If you were to have looked at the 100 tallest buildings in the world in 1969, you would be almost certainly looking at steel buildings that were office function, and they would be in North America, and predominantly in the United States,” Safarik says. 

Now? They are “composite buildings—some combination of both steel and concrete,” he adds. “And the buildings would largely be located in [the] Middle East and Asia, and they would have mixed functions—so that’s how the coin has really flipped over the interceding half century.”