Our tallest buildings elicit all manner of flowery descriptions and grandiose statement, owing to both their scale and symbolism. In the entryway to Dubai's Burj Khalifa, currently the world's highest, quotes such as "the word impossible is not in the leaders' dictionaries" are prominently displayed, a series of Successories for skyscrapers. But it's numbers, not words, that make these structures so inspiring, specifically complicated engineering calculations. While it's dizzying to think of what's required to construct these massive buildings, for many new construction projects, the math has gotten a even more complicated. Consider New York's Empire State Building, a model of classic skyscraper construction from the early 20th century, and 432 Park Avenue, a recently built, slender 96-story luxury tower overlooking Central Park. It's not surprising the newer building is more than 100 feet taller. What's arguably more impressive is the relative footprints at ground level; at its widest point, the Empire State building stretches 424 feet across. 432 Park Avenue was constructed on a 90-foot square lot.
According to Bill Baker, the structural engineer for Skidmore, Owings & Merrill responsible for the Burj Khalifa, the trend towards taller, thinner buildings has presented new spins on old engineering challenges. When the ratio between the height and width of a building goes beyond 8 :1 or 9:1, it becomes increasingly more expensive to construct, since it requires thicker walls and more sophisticated technology to reduce the amount of swaying and shaking caused by the wind (Baker compared today's thinner supertalls to a fishing rod, and making one stand up straight requires much more reinforcement). The height-to-width ratio for 432 Park Avenue is 15:1; to put that in perspective, if you place a standard ruler on its end, it has a ratio of 12:1.
How do engineers continue to make more with less, and build up while also moving in? According to Baker and Stephen DeSimone, Chief Executive of DeSimone Consulting Engineers, a company that has worked on a string of supertalls with thin footprints, it's a matter of wind and weight.
A photo of the facade of China Merchants Tower, located in Shenzhen, China, and a graphic explaining how designers shaped the building, in part, to mitigate wind forces. Photo courtesy SOM / © Tim Griffith. Graphic courtesy SOM.
"Confusing the Wind"
Wind is the "dominant force" in tall buildings, says Baker. Over time, engineers and architects have become more and more sophisticated when it comes to shaping a building to account for gusts that can, on very rare days, reach 100 miles-per-hour at the crown of a 90- or 100-story skyscraper. Early in the design process, different shapes for a proposed tower are workshopped and run through wind tunnel testing to determine which one is most efficient. Computer simulations for complex wind patterns still take a long time, so model testing often works best to determine factors such as lift and cross-breezes. Baker says, "the wind tunnel is a giant calculator."
Skyscraper designers want to "confuse the wind," says Baker. Air pushing against the surface of a tall tower creates vortices, concentrated pockets of force that can shake and vibrate buildings (the technical term is vortex shedding). The aim of any skyscraper design is to break up these vortices. Facades often have rounded, chamfered or notched corners to help break up the wind, and sometimes, open slots are grooves will be added to let wind pass through and vent, in effect disrupting the air flow.
"It's interesting that the aerodynamics of the building are almost counterintuitive," says DeSimone. "We don't want smooth shapes, we want shapes that break up the air flow."
Dampers: Shock Absorbers for Supertalls
To help counter the shifting and swaying of building, engineers also utilize dampers, massive devices that shift and help stabilize tall structures like counterweights. Think of them like the weights in a grandfather clock; engineers attach 300-800 ton pieces of steel or concrete on a floor near the top of a tower, tuning and adjusting chains to balance them so they move out of phase with local wind patterns, steadying the tower. Two main types of dampers are used today; tuned mass dampers, which function like swinging pendulums, and slosh dampers, or slosh tanks, large pools of water that help absorb vibrations. The technology isn't new; it's been used on buildings such as the Seagram Tower, completed in 1958. But it's become more common and more sophisticated. Some tuned mass dampers even use actuators, or small motors, to shift and move in opposition to the wind. The engineers of the Shanghai Tower even devised a damper system with powerful magnets.
According to DeSimone, all this effort to limit the swaying of a building, which can cost upwards of $5 million per project, pays off. Top floors of buildings with these types of systems will only shift two-and-a-half feet during rare, incredibly strong, once a century gusts of wind, an amount that's imperceptible to the naked eye (though it can make people feel seasick).
"We Shouldn't Call It Concrete Anymore
Even with carefully engineered facades and vibration-canceling technology, supertalls still need to support massive amounts of weight. While we haven't moved past concrete and steel, technological advances means the elemental ingredients of skyscrapers can support much larger loads with much less material. "Concrete is amazing these days," says Baker. "We should call it something new, since it's so different than concrete from a few decades ago." More workable and up to five times stronger, concrete today has gained these powers due to a more complex chemical composition. In many cases, industrial by-products, such as fly ash, slag from steel mills and microsilica left over from silicon manufacturing, are added to strengthen the mix, allowing it to be stiffer and support heavier loads.
Baker says that many building engineers are experimenting with composite structures that combine high-strength steel and concrete in different ways (concrete-filled steel tubes, for instance) to find the right balance of strength and flexibility. Where builders may have been limited in the past, stronger materials means they can build taller while maintaining the same size structural elements, according to DeSimone.
The most exciting part about these technical advances is that they promote unique designs. To explain, Baker compares the design process of buildings against that of cars. Since vehicles are all trying to solve a similar engineering issue in regards to wind and aerodynamics, car shapes have tended to move towards a uniform middle, and bear a much closer resemblance than they did decades ago. The opposite is happening with tall buildings; the combination of site-specific environmental factors, and the desire to make each supertall a signature part of a city's skyline, means towers will continue to evolve in different and creative ways.