Sky-High Science

All over the world, people are racing to construct taller and taller buildings. Many of today’s skyscrapers are considered to be supertall—structures that stand more than 300 meters (984 feet) high. Architects and engineers have to take every last detail into account to make sure their towering creations don’t topple. Their buildings must withstand everything from the downward force of gravity to high-speed winds that can come from just about any angle.

Unfortunately, even when architects try to consider every possible force, things can still go wrong. One example is the 279 m (915 ft)-tall Citigroup Center, a New York City skyscraper. A year after it was built, an architecture student discovered that it could collapse if a strong wind hit it from a 45-degree angle. She called the architect who designed the building to alert him to the flaw.

The Citigroup Center had a system of braces, or diagonal steel structures that provide extra support to the building. But something wasn’t quite right. The braces were bolted instead of welded together, making them weaker than expected. To correct the error, the building’s architect had workers weld them together, and the building was saved.

Find out how three other supertall skyscrapers were engineered to safely soar to amazing heights.

All over the world, people are racing to construct taller and taller buildings. Many of today’s skyscrapers are considered to be supertall. These are structures that stand more than 300 meters (984 feet) high. Architects and engineers have to look at every last detail to make sure their towering creations don’t topple. Their buildings must withstand every force that acts against them. This includes the downward force of gravity and high winds that can come from just about any angle.

Architects try to consider every possible force. Even then, things can still go wrong. One example is the 279 m (915 ft)-tall Citigroup Center, a skyscraper in New York City. A year after it was built, an architecture student spotted a problem. The building could collapse if a strong wind hit it from a 45-degree angle. She called the architect who designed the building and told him about the flaw.

The Citigroup Center had a system of braces. These diagonal steel structures provide extra support to the building. But something wasn’t quite right. The braces were bolted instead of welded together. This made them weaker than expected. To fix the mistake, the building’s architect had workers weld them together, and the building was saved.

Other supertall skyscrapers safely soar to amazing heights. Find out how three of them were engineered.

The Burj Khalifa is currently the world’s tallest building, at 828 m (2,717 ft). At 163 floors, it’s almost twice as tall as One World Trade Center in New York City—the tallest building in the U.S. (see World’s Tallest Buildings). The building is located in Dubai in the United Arab Emirates.

The Burj Khalifa contains a whopping 500,000 tons of steel and concrete. The building’s huge mass, combined with the downward pull of gravity, puts incredible amounts of stress on the lower parts of the structure. To support this vertical load, architects situated the Burj Khalifa on a massive foundation. They also gave the base of the building a winged design. Arranged in a big Y formation, three wings connect to a central core. These wings buttress, or support, the core as it rises.

The Burj Khalifa is now the world’s tallest building. It’s 828 m (2,717 ft) high, with 163 floors. That makes it almost twice as tall as One World Trade Center in New York City—the tallest building in the U.S. (see World’s Tallest Buildings). The building is located in Dubai in the United Arab Emirates.

The Burj Khalifa contains 500,000 tons of steel and concrete. The building’s huge mass and the downward pull of gravity make a powerful combination. This puts incredible stress on the lower parts of the structure. Architects needed to find a way to support this vertical load. So they placed the Burj Khalifa on a massive foundation. They also gave the base of the building a winged design. Three wings are set up in a big Y formation. They connect to a central core. These wings buttress, or support, the core as it rises.

“As you go up, each wing is stepped back in a deliberate way,” says Jon Galsworthy, head of the wind engineering group at RWDI, a company based in Canada that worked on the Burj Khalifa. The design reduces the weight of the building at its center and helps disperse powerful winds of up to 100 kilometers (62 miles) per hour.

“As you go up, each wing is stepped back in a deliberate way,” says Jon Galsworthy. He’s head of the wind engineering group at RWDI, a company in Canada that worked on the Burj Khalifa. The design decreases weight in the building’s center. It also helps spread out powerful winds of up to 100 kilometers (62 miles) per hour.

The tallest residential building in the Western Hemisphere towers 96 floors above Central Park in New York City. Its slender, pencil-like form reaches 426 m (1,398 ft) above city streets. Inside are some of the most luxurious—and expensive—apartments ever built.

There’s a $95 million penthouse at the top of 432 Park, but that’s not the real marvel of the building. It’s the engineering that makes it all possible. “432 Park was a particular challenge because its slenderness made it vulnerable to wind,” says Jon Galsworthy, whose engineering firm worked on this building as well as the Burj Khalifa in Dubai. “But the designers wanted it that way, so we had to develop a solution.”

The tallest residential building in the Western Hemisphere sits in New York City. It towers 96 floors above Central Park. Its slender, pencil-like form reaches 426 m (1,398 ft) above city streets. Inside are some of the most luxurious—and expensive—apartments ever built.

A $95 million penthouse is at the top of 432 Park. But the real wonder is the engineering that makes it all possible. Jon Galsworthy’s engineering firm worked on this building too. “432 Park was a particular challenge because its slenderness made it vulnerable to wind,” Galsworthy says. “But the designers wanted it that way, so we had to develop a solution.”

Supertall buildings are meant to sway slightly in the wind, which keeps the buildings from snapping under stress. But too much swaying—more than a foot or so in heavy winds—can cause people inside the building to experience motion sickness.

One solution was to place blow-through floors every dozen stories up the building. These floors are left empty and open to the elements so that wind can pass through. That reduces the wind load—the force caused by wind pushing on the building—and the resulting sway.

A second fix was to install two massive 650-ton tuned-mass dampers near the top of 432 Park. These devices reduce the size of vibrations, such as those caused by wind, on a structure. They have a large weight that hangs from cables and gently swings back and forth like a giant pendulum. They’re designed to move in the opposite direction of the building’s sway to lessen the overall movement.

Supertall buildings are meant to sway slightly in the wind. This keeps them from snapping under stress. But heavy winds can cause too much swaying. More than a foot or so can make people inside the building feel motion sickness.

One solution was to place blow-through floors every dozen stories up the building. These floors are left empty and open to the weather so that wind can pass through them. This reduces the wind load—the force caused by wind pushing on the building. As a result, the building sways less.

Engineers came up with a second fix. They placed two huge 650-ton tuned-mass dampers near the top of 432 Park. These devices reduce the size of vibrations, such as those caused by wind, on a structure. They have a large weight that hangs from cables. It swings gently like a giant pendulum. The devices are built to move in the opposite direction of the building’s sway to lessen the total movement.

The Shanghai Tower in Shanghai, China, is the second-tallest building in the world, standing 632 m (2,073 ft) tall. The 128-floor building’s twist is its most distinctive feature.

While designers chose to include a twist, it was up to engineers to determine what angle the twist should be in order to best withstand strong winds. Engineers tested various designs by placing models of the building inside a wind tunnel. Air blew over and around the models, allowing engineers to measure the buildings’ wind loads. The winning design experienced 24 percent less wind load than the others. Because the building would experience less force from wind, its structure didn’t need to be as strong as other designs. The builders were able to use 25 percent less steel, saving an estimated $58 million.

The Shanghai Tower in Shanghai, China, is the second-tallest building in the world. It stands 632 m (2,073 ft) tall. The 128-floor building’s twist is what really makes it stand out.

Designers chose to include a twist. But the angle was up to engineers. They had to figure out what angle would best withstand strong winds. Engineers tested different designs by placing models of the building inside a wind tunnel. Air blew over and around the models. This allowed engineers to measure the buildings’ wind loads. The winning design experienced 24 percent less wind load than the others. Less force from wind gave this design another advantage. The building’s structure didn’t need to be as strong as other designs. The builders were able to use 25 percent less steel, saving about $58 million.

The ground under the Shanghai Tower presented an additional engineering challenge. The soil there is loosely packed, like sand. When a person walks on the beach, the weight of his or her foot pushes the sand particles together. That compression causes the person’s foot to sink. The same thing would happen with the Shanghai Tower, which weighs tens of thousands of tons.

To prevent the tower from sinking, engineers drove 947 steel piles more than 60 m (197 ft) into the ground. They covered the metal columns with a 6 m (20 ft)-thick layer of concrete. This superstrong foundation helps stabilize and support the building.

“The engineer’s job is to come up with the worst possible scenario and design solutions for it,” says Kathy Dunne, a professor of architecture at Pratt Institute in Brooklyn, New York.

The ground under the Shanghai Tower presented another challenge for engineers. The soil there is loosely packed, like sand. Think of what happens when a person walks on the beach. The weight of his or her foot pushes the sand particles together. That compression causes the person’s foot to sink. The same thing would happen with the Shanghai Tower, which weighs tens of thousands of tons.

Engineers needed to prevent the tower from sinking. They drove 947 steel piles more than 60 m (197 ft) into the ground. They covered these metal columns with a 6 m (20 ft)-thick layer of concrete. This superstrong foundation helps support the building and keep it steady.

Kathy Dunne is a professor of architecture at Pratt Institute in Brooklyn, New York. She says, “The engineer’s job is to come up with the worst possible scenario and design solutions for it.”