World's tallest buildings - includes a chart of tall structures from the Great Pyramid to Shanghai, China's in-progress World Financial Center: includes how to build a model skyscraper

Science World, Oct 4, 1996 by Emily Costello

Two new buildings scrape the sky at world-record-breaking height. What keeps these tall towers from tumbling down?

Talk about being on top of the world! Construction workers in Kuala Lumpur, Malaysia, recently stood a dizzying 427 meters (1,400 feet) above the ground and bolted steel pinnacles to the top of the Petronas Towers. With the pinnacles in place, the towers stand 451.9 meters (1,483 feet) high. That makes the buildings 8.9 meters (29 feet) taller than Chicago s Sears Tower--and the newest tallest buildings in the world!

Designing super tall buildings isn't easy. So how do engineers keep the towers from tumbling down?

BENEATH THE BASEMENT

A solid building begins width a strong foundation--the part of the structure that is underground, says Christian Meyer of Columbia University's School of Engineering. The foundation's job, Meyer explains? is to make sure the soil under the building can support the enormous load, or weight, of the building and its contents.

"A building--especially a tall building--weighs a lot," Meyer says. He's not joking. The Sears Tower weighs about 225,500 tons. The taller Petronas Towers are even heavier.

The best material for supporting that kind of enormous weight is solid rock or bedrock. Bedrock is good at resisting the force of compression created by the weight of the building pushing down. In other words, the rock holds its shape under the force. Result: A building on bedrock won't sink.

But reaching bedrock isn't always easy. To get to the solid rock under New York City's World Trade Center, for instance, workers dug down 21 meters (70 feet) and scooped up, enough dirt and loose rock to build a 23-acre land extension on Manhattan's West Side!

The bedrock under the Petronas Towers is buried even deeper. After workers hauled out enough soil to create a hole 20 meters (65 feet) deep for the buildings' three basements, they still hadn't come close to hitting solid rock.

Removing even more soil would have been expensive. So instead of supporting the building on bedrock, the Petronas Towers engineers took a different approach.

MATERIAL MATTERS

Construction workers dug 210 narrow vertical holes--some as deep as 105 meters (344 feet)--into the soil. Then they filled the holes with reinforced concrete--long steel rods surrounded by concrete, says Udom Hungspruke, one of the structural engineers who helped design the buildings. These long concrete-and-steel tubes are called barrettes. Friction between the barrettes and the soil around them keeps the barrettes in place so they can support the building.

Concrete alone couldn't have done the job, explains Rustum Roy, a materials scientist at Penn State University. Concrete--which is made of small stones, sand, and cement--tends to develop pores, which are weak, Roy explains. Those weak spots make concrete likely to break when exposed to a bending or pulling force called tension. The steel bars in reinforced concrete give the composite material greater strength against tension, because steel has fewer weak spots, Roy says.

FROM THE GROUND UP

Reinforced concrete was also the perfect material for the next layer of the foundation. This 4.5-meter (14.8-foot) thick concrete slab distributes the building's weight evenly to the barrettes.

On top of the slab, workers constructed the columns and floor beams that make up the building's frame, or supporting structure. Most skyscrapers have steel frames. In fact, the invention of steel helped make some of the first tall buildings possible (see timeline, p. 14).

Because steel is very strong, you don't need much of it to support the weight of a building, Meyer says. In New York City's World Trade Center, for example, the exterior columns are made of hollow steel tubes with walls only 7.5 centimeters (3 inches) thick.

But steel also costs a lot. So the Petronas Towers' frame is made of cheaper--and weaker--reinforced concrete, which is easier to get in Malaysia, Hungspruke says. To make up for the difference in strength, the Petronas Towers' builders had to use a lot of reinforced concrete. The towers' largest columns are 2.4 meters (7.8 feet) in diameter and solid. One of the building's engineers compares these massive columns to the trunks of giant redwood trees.

WIND WILLIES

Like trees, tall buildings must be able to sway in the wind. If they didn't, the wind would literally push them over. Still, you don't want to let the building sway too much. The motion might make people who work inside sick!

The concrete in the Petronas Towers makes the frame stiff enough to minimize swaying. And the reinforcing steel helps the building resist the wind's bending force. But the building still moves in a strong wind.

Hungspruke won't say how much. But, in the United States, tall buildings are allowed to sway 1/500th of their height. That means that if the Petronas Towers were built here, the tops of the buildings could sway about one full meter (3 feet) in each direction!

SKY'S THE LIMIT

So, are the Petronas Towers about as tall as buildings can get? Not even close, Meyer says. Reinforced concrete buildings could be about twice as tall as the new record holder. And buildings made with stronger steel frames could I'll be about three times as tall!