Mon November 7, 2011
How The World's Tallest Skyscrapers Work
Originally published on Mon November 7, 2011 2:01 pm
When the Empire State Building was constructed in 1931, it stood 1,250 feet tall. The famous skyscraper was the world's tallest building — and held that title for more than 40 years.
Today the world's tallest building is the Burj Khalifa in Dubai. It stretches more than 1,000 feet above the Empire State Building — 2,717 feet into the air. The Burj Khalifa smashed the record held by Taiwan's Taipei 101, a landmark skyscraper with 101 floors. And at 1,666-feet, Taipei 101 tops the Petronas Towers in Kuala Lumpur by 183 feet.
Designing these massive skyscrapers is a huge challenge for structural engineers. Builders must account for potential natural disasters like earthquakes and hurricanes. They also must take everyday weather occurrences into consideration. Even on a normal day, wind forces can reach more than 100 mph at the very top of very tall buildings.
"There are wind currents that when we're down below [on the ground] we don't even notice," says Kate Ascher. "That building, as it rises, interrupts those flows. The larger the building, the greater the wind flows."
Ascher, the former executive vice president of the New York City Economic Development Corp., explores the inner workings of skyscrapers in her new book, The Heights: Anatomy of a Skyscraper. She tells Fresh Air's Terry Gross that engineers purposefully design buildings to sway back and forth in order to alleviate the pressure caused by these high wind flows.
"If a building weren't able to move at the top, then various structural elements might be damaged because of the wind pressure," she says. "And some, in earthquake zones, will be designed to move a little bit on their foundations as well — so they don't take as much pressure as they would if they were absolutely static."
Ascher explains that the same principle that allows a building to get taller also accounts for its ability to sway. Joints at the corner of steel beams can expand and contract because of changes in the weather or wind, which allows for slight movements at the very top of buildings.
"There's not a precise formula for how much sway a building has, but there is a maximum [amount], which is 1/500 of the building's height," she says. "The minute you get more than that, it's not like the building's going to sway or fall over, but people who are in the buildings themselves will start feeling a little bit queasy."
Some buildings, like the Comcast Center in Philadelphia, are equipped with liquid-filled dampers, which help counteract the sway. The 300,000 gallons of liquid act as a counterweight. China's Shanghai World Financial Center, meanwhile, was built with a hole at the very top of its structure. The aperture is designed to reduce wind pressure by allowing wind to flow freely through it.
In addition to structural concerns, engineers have to worry about the inside of skyscrapers. In a 100-plus story building, circulating clean air and water is not an easy task — particularly when windows don't open. Mechanical areas exist solely to swap inside air with circulating air outside, says Ascher, so you don't have to worry that you're breathing stale air.
"It's a constant process," she says, "and it happens around the clock."
And don't worry about flushing a toilet on the 100th floor, either. Engineers have accounted for that, too, she says.
"There are very sophisticated bends in the pipes to slow the water as it's moving through," she says. "You don't want to hear it as it's moving through the building, so you need to make sure its sound-proofed as well."
TERRY GROSS, host: Skyscrapers are no longer rare, and they're no longer an American phenomenon, but they're still remarkable. As if just staying up weren't amazing enough, as my guest Kate Ascher writes, these giant buildings function as vertical cities, providing infrastructure and services to thousands of people at enormous heights. Her new book, "The Heights: Anatomy of a Skyscraper," explains everything from how skyscrapers are built to what happens after a toilet on a high floor is flushed. Her previous book, "The Works," was about the infrastructure that supports daily life in New York City. She is on the faculty of Columbia University's Graduate School of Architecture, Planning and Preservation, and is a consulting engineer running the urban planning section at Happold Consulting.
Kate Ascher, welcome to FRESH AIR. Kate Ascher, welcome to FRESH AIR. Love the book. I have to tell you, it made me a little nervous because my theory is when you are in a skyscraper, especially if you're on the higher floors, you don't even want to think about how it's designed and how it manages to stay up without collapsing, and how the plumbing works. You just want to have faith - faith that it all stays up, that it all works.
(SOUNDBITE OF LAUGHTER)
GROSS: And reading your book and actually reading about the design, you think like, oh my God, how did they do that? So I'm not even sure what my question is for this, but I guess you know both: You know how it works, and you also have faith it's going to stay up.
KATE ASCHER: I do. But I was like you - not totally comfortable on high floors of skyscrapers, except I would occasionally hear them moving and worry about that movement. And I guess part of the motivation to do the book was actually, to figure out what was going on there and whether that was supposed to be the case or not.
GROSS: Well, while we're on the subject, you talk about how there's two kinds of weight impacting on a skyscraper. One is the weight of one floor on top of the other, on top of the other, on top of the other. You say that's the easy part. The really hard part is all the wind forces that push on the skyscraper from different ends. Why is the wind pressure so difficult an engineering problem?
ASCHER: Well, it's an interesting question that you ask because the gravity loads - which one assumes are the heaviest weights upon a building - as a building gets taller, don't multiply nearly as fast as what are called the lateral loads, or the wind loads, as you note. And that's really a function of just the size and the height as you move up because there are wind currents that when we're down below, we just don't even notice. And that building, as it rises, interrupts those flows, and they push harder and harder. The larger the building, the greater the wind flows. You can moderate them, in some ways, by shaping your building to take advantage of those flows, better or worse. But they're still going to be there.
GROSS: So skyscrapers are built to sway back and forth with the wind. Why does swaying help?
ASCHER: If a building weren't able to move at the top, than various structural elements might be damaged because of the wind pressure. So most skyscrapers will be designed to move at the top. And some, particularly in earthquake zones, will actually be designed to be able to move a little bit on their foundations as well, so they don't take as much pressure as they would do if they were absolutely firm and static.
GROSS: Say on the hundredth story of a skyscraper, how much sway is there?
ASCHER: It's not a precise formula, in terms of the amount of sway in a building. But there is a maximum, and it's about one-500th of the building's height is the maximum permitted sway. The minute you get more of that, it's not that the building is going to crack or fall over, it's that people who are actually in the building themselves - as tenants or residents - will start feeling a little bit queasy.
GROSS: Now you describe the Comcast Center, a pretty new skyscraper in Philadelphia, has the largest tuned liquid damper to do what?
ASCHER: The dampers on top of buildings help counteract the sway. So if you imagine that the wind is pushing a skyscraper in one direction, a damper - whether it's water or made of some other material - will push the building the other way, to try to counteract the sway that's coming from the wind.
GROSS: I'm trying to picture how it would do that.
ASCHER: Well, if you think about a tank of water - because the liquid dampers are really tanks of water, or a pool on top of the building - as the wind comes from one direction, it will push the will - building in that direction but the water, of course, will go to the other side. So that weight of water moving to the other side, almost like a wave, will counteract the wind pressure and help the building stay in one place, as it were.
GROSS: One of the great like, architectural paradoxes of our time is that these skyscrapers that have like, all glass sides and it looks like just window after window, the windows don't open.
(SOUNDBITE OF LAUGHTER)
GROSS: So they don't really function in that way as windows. So if you're in this, you know, like, huge skyscraper of 100 stories or more, how do you get air to circulate through the building and keep that air fresh and not just re-circulate, like, horrible air with lots of bacteria and germs and other terrible things?
ASCHER: Right. You can imagine - if you didn't circulate the air, you'd have people getting sick very, very quickly. There are huge mechanical floors on these skyscrapers that really do nothing but change out air and water. And that air is continuously being changed out and exhausted into the environment. So air is taken in, cleaned, conditioned, cooled, heated, and then exhausted into the building. There are some new buildings now that are actually exhausting cleaner air than the air they take in. But it's a constant process, and it happens around the clock.
GROSS: So let me ask a question that I think everybody really wants to know. When you're on the 100th floor of a building and you flush the toilet, what exactly happens after that?
ASCHER: It's not that different than a house. The water from the toilet goes down a series of pipes and eventually will end up in some kind of septic system, usually a municipal sewer. But it's the same system that operates in many homes that are plugged into city sewer systems. It's just a longer way for the waste to travel.
GROSS: Well, if waste travels like, 100 stories, isn't like – doesn't it start to travel with incredible speed? Wouldn't it speed up, the greater distance it's falling down?
ASCHER: It does. There are very sophisticated bends in the pipes and air that's let in to slow the water as it's moving through. And you don't want to hear it as it's moving through the building, so you need to make sure that it's soundproofed as well. And these mechanical...
GROSS: I hadn't even thought of that.
ASCHER: Right. Well, you've probably heard of radiators banging. That's water moving through the pipes in buildings for heat. And so you can have the same issue that's water moving out of a building, so you really want to make sure of that. There are some buildings that actually reuse what's called gray water or dirty water, and that's increasingly being seen in what are called green buildings, to minimize water usage - not for drinking, but for other functions within a building. But typically, in most skyscrapers, that drainage goes right into a city system.
GROSS: You write that there are safety features in other countries that are better than ours, and that our skyscrapers have fewer safety regulations - or more lenient safety regulations that they have to comply with. Give us a sense of what's being done in some other countries, safety-wise, that we're not doing.
ASCHER: Well, there's different materials in different places, for a start. But there are also escape floors that - they're also called floors of refuge, in many of the Asian buildings, and they're places where people are asked to go in a fire or other kind of emergency, as opposed to coming all the way out of the building. They're asked to assemble at the refuge floors, and then there are sometimes express elevators that will take them from those floors.
Or they're asked to stay there until the emergency has passed. And so you have separate air systems for those buildings so that the people can, theoretically, survive in an emergency instead of trying to get all the way out of a very tall building - because, as we know from the Trade Center experience, it can take hours for a really tall skyscraper to empty itself of its people.
GROSS: And so compare that to what we typically have in American skyscrapers.
ASCHER: Well, in American skyscrapers there are building codes that, you know, explain how many stairways you need to have, and the width of stairways and the lighting of stairways. There are also procedures for evacuating them, and people do fire and emergency drills all the time. But at the moment, when there is a fire or other emergency in a skyscraper, the typical reaction is not to use the elevators; it's simply to assemble and to have an orderly move-out based on whoever is controlling that evacuation.
Not everybody needs to evacuate but certainly, the floors above a fire are usually asked to evacuate. Different cities have different procedures but typically, elevators are not used in a fire situation to evacuate people whereas they are – there are separate sets of elevators that may be used to do that in Europe and in parts of Asia.
GROSS: So what is currently the tallest building in the world?
ASCHER: The tallest building in the world is a mixed-use building that opened in Dubai last year, called the Burj Khalifa, which holds hotel, office and residential accommodation.
GROSS: And how many stories is that?
ASCHER: It's about 140 stories, I believe. It may be 160. It's about 2,700 feet tall so it's very, very tall.
GROSS: Have you been in it?
ASCHER: I haven't been in it. The last time I was in Dubai, it was only 115 stories, but I couldn't see the top of it because it was in cloud. So I'm looking forward to going back and seeing if I can make it on the observation deck elevators all the way to the top - which for somebody who doesn't like heights, this should prove quite a challenge.
GROSS: Right. So you know, you write that in Dubai they don't have like, a sewage infrastructure to support high-rises like this one. So what do they do with the sewage?
ASCHER: A variety of buildings there; some can access a municipal system, but many of them actually use trucks to take the sewage out of individual buildings. And then they wait on a queue to put it into a wastewater treatment plant. So it's a fairly primitive system.
GROSS: Well, these trucks can wait for hours and hours on line.
ASCHER: That's right. I'm told they can wait up to 24 hours before they get to the head of the queue. Now, there is a municipal system that is being invested in, and I assume will connect all of these tall buildings at some point in the near future. But they're certainly not alone. In India, many buildings are responsible for providing their own water and their own wastewater removal.
So it's, it's really – we're very fortunate in this country that we assume we can plug into an urban system that can handle whatever waste the building produces. That's not the case everywhere else in the world.
GROSS: Well, it really illustrates one of the paradoxes of modern life, that we have these just incredible structures that reach, you know, that seem to reach to the sky. And then in a place like Dubai, you have a 24-hour-long line of trucks waiting to dispose of the waste from those buildings.
ASCHER: Right. Well, you know, you have to remember that a place like Dubai really emerged in the last 50 years. It was a sleepy, you know, Bedouin town half a century ago. And what you do is when you bring in the world's, you know, most sophisticated architects and engineers, you can literally build anything, including a building of 140 or 150 stories. But designing a municipal network of sewage treatment is, in some ways, more complex - certainly requires more money and more time to make it happen. So one just seemed to jump ahead of the other.
GROSS: So there are new skyscrapers now that are mixed-use skyscrapers, so they combine offices, residential space, stores. So what are some of the stresses on a building like that, compared to the more traditional - just like, office skyscraper?
ASCHER: It's a little bit harder to design those buildings because what you have is different flows of people coming in at different times of day who don't always want to run into each other. So you'll often have a premium hotel that might sit on top of an office block with a variety of retail shops down below, and you don't want your hotel guests - particularly if they're being charged a pretty penny - running into your workaday folks coming in and out of the office at lunchtime.
And so you really need to segregate entrances and exits. You really need to make sure that not everybody's moving in and out at the same time of day because it can also get very, very crowded. And then you'll also have residential accommodation at higher levels in some of these buildings, and those folks use a different schedule entirely.
So you have different entrances, and you have different elevatoring because you have different elevator systems that are going to move to serve each of those markets. So it's a very complicated design process.
GROSS: And then just technically, there's a different amount of floor space that's required for office buildings and for residential space.
ASCHER: Right. And they tend to often work together. Offices tend to be bigger and need to be more open, so with less walls because you need, you know, space for conference rooms and various things which you don't have in homes. Homes, of course, you want as much window exposure as possible because people don't want to live in rooms without windows. So they tend to have smaller footprints, or floor plates.
So what you'll frequently see nowadays is an office podium down below, a residential tower on top - with the tower being skinnier, of course, than the podium office because you need to have those windows very close to each room. And that seems to be the most popular configuration at the moment.
GROSS: Kate Ascher, thank you so much.
ASCHER: Thank you.
GROSS: Kate Ascher is the author of the new book "The Heights: Anatomy of a Skyscraper." You can see a slideshow of images from her book on our website, freshair.npr.org, where you can also download podcasts of our show. And you can follow us on Twitter at nrpfreshair, and join us on Facebook. Transcript provided by NPR, Copyright NPR.