How To Spot The Engineering Tricks Hidden In Buildings
The gentle curve of a beam. The particular shape of a clay brick. The sharp angles of a series of trusses. You might view these elements of buildings, bridges, and structures as part of the aesthetic and artistic design, or maybe you have overlooked them completely. But for London-based structural engineer Roma Agrawal, these visual charms play an important role not only in the beauty of a building, but in the physics that keep a structure from tumbling down.
In her new book Built: The Hidden Stories Behind Our Structures, Agrawal explores the evolution of structural engineering—from ancient massive domes that do not collapse under the force of their own weight to modern skyscrapers that defy the laws of gravity. As Agrawal explains, if you strip away the layers of a building—all the way down to its guts—you can find the engineering tricks that go into the infrastructure. She joins Ira to reveal some of the hidden engineering, materials science, chemistry, and physics that help structures support their own weight.
[Read an excerpt of Roma Agrawal’s book Built.]
You can try spotting these elements yourself! Check out some of the clever, inconspicuous building components that have helped engineers pull off amazing structural feats.
The dark, 100-story John Hancock Tower stands tall among Chicago’s skyline—the cuboid skyscraper known for its bold, black X-latticed exterior. But the iconic design also serves as an exoskeleton that braces the tower and reduces the lateral load by transferring it into the exterior columns. It was designed by architect Fazlur Rahman Khan in 1965.
The most recognizable feature in San Francisco’s famous suspension bridge is probably the two towers that peek above the fog on the bay. If you look closely at the pattern on the sides of the bridge, however, you’ll notice a geometric sequence along the entire length of the bridge. This network of trusses help with distributing force. The mirrored N-shape pattern allows for less material making it lighter weight—crucial for constructing bridges that span over long distances.
The Burj Khalifa is the tallest man-made structure in the world, topping off at 2,700 feet tall. How did engineers construct the Dubai megatall skyscraper? A series of tubes—and no, not the Internet. Just like how multiple plastic straws clustered together are much stronger than a single straw standing on its own, the Burj Khalifa is comprised of a series of tubes that each have their own exoskeleton, Agrawal explains. There are more shorter tubes at the bottom of the structure which reinforce the tubes that escalate to the tallest point. From above, the tubular structure takes the shape of a blooming flower, while also making sure the tall building is as stable as possible.
Similar to the Hancock Tower, the pickle-shaped 30 St Mary Axe in London, popularly known as the Gherkin, also has a unique exoskeleton. The blue-green tinted glass is interlaced with a series of diamonds, the large criss-crossing beams of steel forming a diagrid. The Gherkin’s exoskeleton diagrid is “like the shell of a turtle,” Agrawal writes in her book. It resists and distributes forces down to the foundations, as well as helps with the sway of the building.
The Romans were true masters of concrete, and the Pantheon is an exemplary model of that craftsmanship. The interior of the Pantheon’s concrete dome, with its oculus casting an ethereal ray of sunlight on the marble floor, is a breathtaking sight. But one important trick that maintains the integrity of the dome can actually only be seen from the outside of the building. In addition to making the dome thicker at the base, the Romans also added seven steps of concrete that circle around the base. These steps, which reach about halfway up the dome, help resist some of the tension forces, Agrawal explains in her book. The concentric steps are also seen in other domed buildings, like Columbia University’s Low Memorial Library.
Roma Agrawal is a structural engineer and author of Built: The Hidden Stories Behind Our Structures (Bloomsbury USA).
IRA FLATOW: This is Science Friday. I’m Ira Flatow. If you’re a regular listener to this program, you may have heard me mention more than a few times how much I love talking and thinking about concrete, yes, concrete. Yeah, I know, cement turning into concrete.
So when I read our next guest’s book with the opening line of a chapter reading, “I’ve been known to stroke concrete,” I knew I had found a kindred spirit. Roma Agrawal is a structural engineer who’s helped design towering skyscrapers, including the Shard. That’s that 95-story tower in London.
And in her new book, Built: The Hidden Stories Behind Our Structures, Roma shares the secrets behind how skyscrapers are built, why ancient Romans were the masters of concrete, and one of my favorite stories, how John Roebling may have designed the Brooklyn Bridge, but his daughter-in-law Emily, with no formal education in engineering, was the driving force behind getting it built, sort of a hidden figure, right? So if you’ve been wondering why buildings, bridges, sewer systems are all built the way they are, or you look around, you’re outside, and you say, hey, I see this building going up. What’s this piece do with– what does that do?
Give us a call, (844) 724-8255. You can also tweet us @scifri. And (844) SciTalk is our phone number. Roma Agrawal joins me from London. Welcome to Science Friday.
ROMA AGRAWAL: Thank you very much.
IRA FLATOW: You know, I’m so happy to talk with you because I’m a fellow geek about construction and structural engineering. And as I said, when I saw the way you described concrete, I knew we were going to bond immediately, so to speak.
ROMA AGRAWAL: I’m so excited that I found someone else that’s as obsessed with concrete as I am.
IRA FLATOW: Well, I’m glad because we could talk all day. But I will have to go into a lot of different directions. First tell me what’s– why is it such an amazing substance, concrete?
ROMA AGRAWAL: So what I really like about it is that it has so many different forms. It’s quite an indeterminate material. So it starts off as being a rock. We then kind of break it down. We heat it up. We fire it.
We then turn it into a liquid. And then that liquid is poured into any shape that you could possibly want. And then it starts to solidify, and you get different textures from it, different colors. And so I just love the fact that it can be anything you want it to be.
IRA FLATOW: And I had heard that it takes months and years to set and dry. It never really totally dries out. Is that true?
ROMA AGRAWAL: Right. So concrete, interestingly, needs a lot of water, so it doesn’t actually lose the water. But it reacts with the water. And that process, most of it happens in the first 28 days of it being poured. But you’re right, it actually takes months and years until concrete reaches its full complete strength and that chemical reaction is completely finished.
IRA FLATOW: And I still can’t wrap my head around the fact that we can pour concrete and let it dry underwater, which you describe in your book. That just blows my mind about that.
ROMA AGRAWAL: It’s amazing, isn’t it? So it was all about finding that special ash, which is what the Romans did. They found pozzolanic ash near their volcanoes. And they started mixing that into their concrete. And they found that, yeah, you it sets underwater because it basically doesn’t need air for that reaction to happen in that particular mixture.
IRA FLATOW: Speaking of concrete, we are already getting a lot of questions from our audience. Let me– while we’re on that subject, let me get a tweet from [? Christophero ?] who says, why is it that the coliseums still stands and yet our highways are falling apart every few years?
ROMA AGRAWAL: That’s a really interesting question. So I think what’s really interesting about old structures is obviously they didn’t have the kind of computing power that we have today. So when they built their structures, they put lots and lots and lots of material in there to make sure it was safe.
And then obviously we’ve been looking after our historic structures pretty well. So combined with the fact that there’s lots of material, and we look after stuff really well, a lot of these ancient structures have lasted. And I expect with highways and so on, they get so much wear from all these trucks and heavy vehicles driving over them, that they just probably needed a little bit more love.
IRA FLATOW: But there wear a lot better than asphalt does, don’t they?
ROMA AGRAWAL: Yeah. I mean, it depends. And we know what the design life is for these various things. So we can’t make our roads last forever.
There’s a finite life for them. That makes sense financially, and how long it takes to build them, and so on. And so it is normal that they do need maintenance. So that’s what it’s about is going in and looking after things better.
IRA FLATOW: Now one of the first projects as an engineer that you worked on was that 95-story tower in London, the Shard, which it looks exactly like the name. What were the challenges of making something that tall?
ROMA AGRAWAL: So it was such a fun project to work on. I think I’ll pick two challenges. One is the ground.
So unlike Manhattan where you’ve got this lovely strong rock that you can build your beautiful towers on, here in London we’re on the banks of a river, so we’ve got this kind of wet, soft clay. So building the foundations was challenge one. And what we do is we put these big huge concrete piles into the ground, they’re big giant concrete columns, to basically anchor it like the tree roots do for the tree, and then it keeps the tower stable. So I’d say ground is kind of big challenge number one.
And the second one that I love talking about is the wind. So we think, oh, the wind is quite harmless. We like a nice little breeze. But they can play havoc with skyscrapers. So we need to make sure that the buildings are stable when wind hits it from all the different directions.
IRA FLATOW: That is a challenge. Our number (844) 724-8255. Boy, how fast this phone line has filled in. Let’s go to Jersey City, or “Joisey.” And hi Matt, welcome to Science Friday.
MATT: Hey guys. My father was a project manager at One World Trade. And he always spoke about how they needed to account for seismic activity from terrorist attacks. And my question is as these buildings get taller, is that more difficult to account for is man-made disasters and things of that sort?
IRA FLATOW: Good question. Thanks for calling.
ROMA AGRAWAL: No thanks very much for that, Matt. Yes, it is something that we do need to think about. So we do a lot of assessments to figure out which ones of our buildings we think are most vulnerable.
We had an example in the UK which in the 1960s actually had a catastrophic collapse because of a gas cylinder explosion. So it’s not just about intentional explosions, it’s also about accidental explosions that might happen. So it’s definitely something that we need to think about. And there’s various techniques that we can use to try and mitigate or minimize the amount of damage that happens.
IRA FLATOW: I watch new construction here all the time in New York. It’s around us every place. There’s a giant building going up right next to Grand Central I’ve watched from day one.
And I watch steel coming down the street. First thing I notice about the steel I beams is that they’re rusted even before they go in. Is that on purpose?
ROMA AGRAWAL: So because the steel beams are ultimately going to end up in our buildings, which are air conditioned, and temperature controlled, and humidity controlled, so yes they do get a little bit of rust on the very surface. But that rust actually, as long as there isn’t more moisture and oxygen, going to affect it. That actually protects the steel a little bit.
So it’s fine for it to have a little bit of rust, and then it goes into our buildings, and then it doesn’t rust anymore because of the temperature control. So it is quite normal. It is counterintuitive. But yes, our steel beams and columns may come to our buildings slightly rusted.
IRA FLATOW: Now when you build a building 60, 70, 80, 100 stories tall, how do you keep it from just squashing the bottom members? I mean, it’s so heavy. How can it be held up? I mean, the rest of the building stay up like that?
ROMA AGRAWAL: Yeah, it’s incredible. And this– all of this comes down to getting the right materials. So we have a lot of experience now. We’ve got computing power. So we can do a lot of mathematical analysis to understand how heavy the building itself is, but also all the stuff that’s going inside the buildings like people, like books in a library, or what else. And we basically crunch the numbers. And then we make sure that the base of the columns are the right material and there’s enough material there to resist those forces.
IRA FLATOW: When my– years ago, my father used to work on the World Trade Center up in the ’80s and that floors up there. And he used to talk about feeling the building sway just a little bit. You’ve talked about that, that you will you allow the building to sway so it doesn’t break, but not enough to make people nauseous. That goes into the calculation.
ROMA AGRAWAL: It does. Isn’t that incredible? So we have an idea for what humans can perceive. What is the acceleration of a building that make us feel a little bit queasy? So when we do our analysis, and we’re looking at how much the building is moving, but more importantly how quickly the building is moving, we try and make sure that that movement is slower than we can really perceive well that makes us feel nauseous.
IRA FLATOW: Let’s go to the phones. Let’s go to Pete in Nyack, where they are building one of my favorite bridges. And I’ve been watching the Tappan Zee Bridge going up. I mean, we’ll talk about what it’s made of. Hi, welcome to Science Friday.
PETE: Hi. That bridge is pretty amazing, isn’t it?
IRA FLATOW: It is amazing, yeah.
PETE: My question is specifically about concrete and the steel reinforcing bars that I know, short-term help to strengthen the concrete. But I think long-term is still reinforcing inside the concrete helps to force it apart. Is that something your guest can speak to?
IRA FLATOW: Sure.
ROMA AGRAWAL: Thanks for the question, Pete. So those steel bars actually do a lot of work. And they’re working hard for the entire life of the structure. So where things can start to go wrong, where those steel bars might actually split the concrete is if they rust.
So we’ve just been talking about rust. We don’t want the steel bars inside concrete to rust. So that’s about making sure that water and air can’t reach that steel. So we need to embed it deep enough into the concrete so that it’s properly protected, and then they’re going to work the entire life of that concrete structure.
IRA FLATOW: Number (844) 724-8255. Let’s talk about appreciation of a lot older structures. Someone called up and asked about the Colosseum. Romans were the masters of concrete. And you know some of the tricks they used to make buildings like the Pantheon both beautiful and strong. Tell us about some of those.
ROMA AGRAWAL: Yeah, the Pentagon is probably my favorite structure in the entire world. And I’ve been lucky enough to visit Rome twice and see it. And again, it comes down to material, it comes down to concrete. But the Romans had, I think, this amazingly can do attitude about construction as well.
And they just built. They built big. They built complicated. They tried new materials. They were really, really experimental. And I really, really admire that can do attitude that they showed.
IRA FLATOW: We’re talking– you mentioned the Brooklyn Bridge, which I want to get to a little bit later. But it was one of the great span bridges, a lot of different bridges like that. But now we see all the new bridges are whether– they have a different kind of suspension. They’re a very stiff looking suspension instead of the beautiful arching there. What are they– what do you call them, stage suspension?
ROMA AGRAWAL: Yeah, the suspension bridges, yeah.
IRA FLATOW: And why change the design? What is better or different about the new ones than the old ones? Why change it?
ROMA AGRAWAL: So I think we still use a lot of the same design techniques and materials, in fact. So the Brooklyn Bridge was in fact the first building in the world that used steel cables rather than iron, which was quite usual at the time. And I think what’s probably changing our structures a little bit is the fact that we’re getting longer, and longer, and longer. So you can use kind of the old techniques for spans up to a certain point. But if you’re really going to push the boundaries on longer bridges, then we need to start looking at slightly different designs for them.
IRA FLATOW: Before the break, I want to make sure we talk about Emily Roebling and her role. I mean, most people don’t know of her role. Although, as you show in the book, there’s a little plaque on the Brooklyn Bridge, most people think that it was her father or her husband that was most crucial in getting this done, but it was Emily.
ROMA AGRAWAL: It was. And she’s such a heroine of mine. I love the fact that she was a woman in an era where people genuinely believed that women were intellectually inferior to men. She couldn’t get a degree in engineering. But she was basically pushed into this situation through the tragic demise of her father-in-law and then the accident her husband faced on site.
But she went in, and what I really particularly admire, is that not only did she learn all the technical skills you need as an engineer, but Washington Roebling, her husband, said her biggest contribution to the build was her talent as a peacemaker. And that is such an important part of engineering structures.
IRA FLATOW: Our number, (844) 724-8255. Let’s see if we can get another phone call or two before the break. See if we can get that one. No, I can’t get that one. Let me just get a tweet in.
A lot of people– there are so many of them. How efficient and long lasting is mud mortar as a binding agent? It’s a very old one, right?
ROMA AGRAWAL: Yeah. So mud– we’ve used mud. We’ve used mortars that are made of gypsum, that are made of lime. And I even talk about in my book how the Chinese actually used sticky rice in their mixture for mortar.
So it really depends on what the mix was. Some lime mortars that are used in the Tower of London, for example, are over 900 years old, and they’ve lasted. There’s mortar between the giant stones of the pyramids in Egypt, and those have done incredibly well. But there are, of course, other structures where the mortars would have worn away over time. So it really depends on the skill and the mix of the substance that the engineers put together at the time.
IRA FLATOW: Are there secrets that we have lost over the years in construction that we can benefit from?
ROMA AGRAWAL: Oh, I’m sure there are. But I think, again, we’re talking a lot about concrete. But let’s talk about it more.
So concrete– we lost the art of making concrete, at least in the West, for about a thousand years after the Roman Empire collapsed. And I just– I wonder sometimes if– we have these amazing Gothic cathedrals in Europe that are made from stone. And I sometimes wonder what our architecture and our engineering would have looked like if we actually had managed to preserve that knowledge of concrete instead.
IRA FLATOW: This is Science Friday from PRI, Public Radio International, talking with Roma Agrawal, author of Built: The Hidden Stories Behind Our Structures. It’s a wonderful book, Ms. Agrawal. And you can read the passion that you have for your subject material in there. Did you always know you wanted to be a structural engineer?
ROMA AGRAWAL: So, the simple answer to that is absolutely not. I always knew that I loved science. I was pretty good at physics and math, in particular. I also enjoyed design. So I studied design technology in the equivalent of high school in London.
And I tried to then have a think about, well, how can I bring all of these passions together? So I actually studied physics as an undergraduate degree. But then I thought about, well, how do I apply that physics to make real objects? And I found out that engineering was the answer. So I was about 20 or 21 years old when I decided I wanted to be an engineer, which just pretty late for a lot of people that have decided, you know, aged five that they want to be an engineer.
IRA FLATOW: Let me see if I can get a quick call from Steven in the Bronx. Hi Steven.
STEVEN: Hi. How’s it going?
IRA FLATOW: Hi there. Go ahead.
STEVEN: So I study art history. So my question is now that things like concrete, and infrastructure, and buildings have become so commercialized, has there have been a loss of attention to aesthetic qualities of infrastructure, and buildings, and stuff like that and more of a focus on their functional, practical purposes?
IRA FLATOW: Yeah, good question.
ROMA AGRAWAL: That’s an absolutely brilliant question. So I think there’s a balance between the two. So I think– so where we need to build lots of good quality infrastructure, lots of good quality housing, and we need to do that reasonably quickly, I think quality and being able to build things efficiently are, probably rightly, the main driver.
But we always still have those beautiful signature projects. So I’m thinking Olympic stadiums. I’m thinking the kind of beautiful big skyscrapers or the signature bridges. So I think it really depends on what the context of the structure is.
But you may not have visited the Shard because it’s quite far away from you guys, but we paid so much attention to every single bolt and weld at the top of the tower, which is about the viewing gallery is because the visitors come in and you can see all the steel. It’s all completely exposed. So there’s the right time and place to pay full attention to the aesthetics and so on.
IRA FLATOW: Well, we’ll have more talk with Roma Agrawal, author of Built: The Hidden Stories Behind Our Structures after the break, more questions. Stay with us. We’ll be right back.
This is Science Friday. I’m Ira Flatow. We’ve been talking about the stories behind the bridges, and the towers, and the other structures we might take for granted. My guest is Roma Agrawal, structural engineer, who’s worked on one of the tallest buildings in the UK.
And she’s the author of a new book that tries to make engineering more visible to the rest of us. It’s Built– book is Built: The Hidden Stories Behind Our Structures. And you can see some of the structures we’ve been talking about and the engineering tricks involved on our website at sciencefriday.com/built. Roma, let’s talk about climate change. Is it forcing new kinds of decisions when we design large structures?
ROMA AGRAWAL: So climate change is such an interesting topic. It makes us more conscious about the sorts of materials that we’re using. So the most man-used, man-made material on our planet is, once again, concretes.
And concrete does emit a lot of carbon, so we’re thinking about how can we make concrete more eco-friendly. We are actually recycling a huge amount of the steel that we now use. So almost 95% of steel can be recycled and reused, which is fantastic. So that’s one of the key drivers for us.
When you think about buildings more broadly, then we’re thinking about energy consumption because our buildings use a lot, a lot of energy. So we’re also thinking about how can we insulate them better, what kind of cladding can we use– can we use more efficient, you know, air conditioning and so on. So there’s lots of different angles that we need to look at from a building point of view. But then, of course, when you’re thinking about cities and so on, then that’s a whole other level of consideration.
IRA FLATOW: Let’s go to the phones. Let’s go to San Francisco. Richard, welcome to Science Friday.
IRA FLATOW: Hi there. Go ahead.
RICHARD: As you said, I live in San Francisco. We have a high-rise building in San Francisco that’s located downtown. And it’s managed to settle well over a foot. And I’m wondering if– or what steps your guest would take if she was designing a building that was to be erected on conditions like we have in San Francisco such as a lot of mud and fill from landfill that has taken place in the past?
IRA FLATOW: Good question. Ms. Agrawal?
ROMA AGRAWAL: Yes, thank you. So I was actually in San Francisco almost exactly a year ago now. And I really loved your city, so great to talk about it. So I think your ground sounds quite similar to London. So we need to use piles, which is what I mentioned earlier, so these giant concrete columns that we install inside the ground.
And the piles are very interesting because they actually work in two ways. So one is with friction. So you get the surface friction between the concrete face and the mud that it’s installed in.
And also the actual very bottom, the base of the pile, can just push directly into the ground as well. So between these two different kind of physical effects, they should be supporting our structures really well. So the settlement shouldn’t really happen to be honest.
But I talk about Mexico City in my book. And Mexico City was actually built– so the center of Mexico City as it is now was built over a lake. And over there, they’ve had settlement of about– the equivalent of three stories in the last 150 years, which is absolutely fascinating.
IRA FLATOW: Yeah, you know and it is fascinating you talk about a lot of hydraulics issues there. Getting back to the piles, and then here in major cities, we watch pile drivers working all the time sinking piles in. But I’ve always wondered in a town– in an island like Manhattan, where there’s so much bedrock, and they’re looking for bedrock, how do you put piles, or steel, or concrete into the bedrock?
I mean, does it lie on top of it? Do you dig a hole in the bedrock? Why is that so important? How does it actually happen?
ROMA AGRAWAL: So from what I understand, you don’t really need to go into the bedrock because the bedrock’s really, really strong, and it’s not going to really let the buildings go anywhere. So all you really need to do is find the top of the bedrock or a nice strong layer of the bedrock and you need to put your concrete foundations just onto there. And then after that, nature does its thing. So it’s actually a lot easier to build skyscrapers in Manhattan, say, than presumably in San Francisco or in London, in fact.
IRA FLATOW: Over the years, you’ve seen– I’ve seen so many bridge structures. And you don’t see rivetors anymore. They don’t show people putting rivets, a lot of bolts. Why do they have so many bolts and rivets in a steel bridge? What is that? What the use of that?
ROMA AGRAWAL: Yeah, so I love that.
IRA FLATOW: Yeah, It’s beautiful to look at.
ROMA AGRAWAL: They’re fantastic, aren’t they?
IRA FLATOW: Yeah.
ROMA AGRAWAL: They have this old world nostalgia, I guess, about them. So you’re right. We don’t really use that technique much anymore. So a lot of the structures which had the rivets at the time were made from iron. And iron is a cousin of steel, but it’s got slightly different properties.
So it needs more connection. It needs a bit more strength than steel does. And that’s part of the reason we’ve moved to steel because it’s a better material to build with. But at the time, the rivets were a really effective way of joining different pieces of iron together. So that change of the base material led to a change in the way we actually joined the metal up together.
IRA FLATOW: All right. In the few minutes we have left, tell us what– your eye view of walking around the city and looking at stuff, what should we look at structurally to be impressed and be amazed by how buildings are erected?
ROMA AGRAWAL: Right. So this is one of my main aims for my book. And I say I want everyone to look at our world through the eyes of an engineer. So what I do when I go up to the viewing gallery in the Shard, for example, everyone’s taking photographs of the river and of St. Paul’s Cathedral. I’m looking up at the steel. And I’m looking at the boats and the welds. And I’m thinking about, well, how– I can see how it was put together.
So walking around Manhattan, for example, you look up at the towers and I guess try and peel away the layers. Think about the glass that’s clouding the structure. How was that glass made? How is it flat enough that the reflections on the light actually traveling through the glass make sure that it’s quite clear and that it’s flat? And what lies behind that? What’s actually supporting that glass?
But then think about trying to use some kind of x-ray vision to look below your feet as well. So I talk a little bit about the sewers of London in my book because they’re absolutely fascinating structures. So you’re walking around London or Paris, and below you are these absolutely stunning brick structures that carry all the waste of these incredibly large cities.
So I feel like you should be looking for peculiar details. You should be looking for the materials. But you should also very much try and look beyond what you can actually see and try and delve deeper into our structures.
IRA FLATOW: Well, I agree with you. The beauty is in the details. And there are plenty of beautiful details in Built: The Hidden Stories Behind Our Structures, written so well by Roma Agrawal, out now. Thank you for taking time to be with us today. And I hope you have another book in you there because it’s so nicely written. Love to read more about–
ROMA AGRAWAL: Oh, thank you, thank you so much.
IRA FLATOW: –structural design.
ROMA AGRAWAL: I will definitely consider that.
IRA FLATOW: Thank you for taking time to be with us today.
Christie Taylor is a producer for Science Friday. Her day involves diligent research, too many phone calls for an introvert, and asking scientists if they have any audio of that narwhal heartbeat.
Lauren J. Young was Science Friday’s digital producer. When she’s not shelving books as a library assistant, she’s adding to her impressive Pez dispenser collection.