by Roma Agrawal
The key to building higher is stabilising the structure externally rather than internally. Perhaps the most precarious experience I can think of was the only skiing trip I ever went on. At first, our instructor wouldn’t let us use ski poles, so I had to stop myself from falling over using just my feet. I soon lost count of the number of times I fell over, and the number of bruises I picked up in the process, but once I had managed to stand upright – at least for a short while – I was allowed the poles. And what a huge difference they made: by spreading my arms out and using the poles to stabilise myself, I found I could stay upright longer. Although the poles were much skinnier and less stiff than my legs, by putting them further apart than my feet could reach, I was more stable.
Tall towers with exoskeletons work in the same way: by spreading the stability from a small internal area (analogous to my feet or a building’s core) to an outside area (the poles or the exoskeleton), it’s possible to create much more stable buildings. Flipping the structure around in this way opened up a number of engineering possibilities: if you built a tower of 50 or 60 storeys like engineers did at the turn of the twentieth century, you could use much less material, making it cheaper. Or if you used the same amount of material as in the older towers, you could build much taller. So, from the 1970s onwards, scores of tubular towers arose, from Hong Kong’s Bank of China Tower and the original World Trade Center Towers in New York to the Petronas Towers in Kuala Lumpur, changing the face of our skylines forever and creating the classic modern-city silhouette.
*
With the invention of new building techniques, structural systems, and computing power increasing every year, it’s an exciting time to be a structural engineer. Just as the height of buildings has increased on the back of what we’ve learnt from our predecessors, so too has the depth of our knowledge. Today, I can design structures that brilliant thinkers like Leonardo da Vinci struggled with. And in a hundred years, engineers will no doubt find it easy to do things that I struggle with now. My peers and I are building on thousands of years of engineering gifted to us by Archimedes, Brunelleschi, Otis, Khan and countless others.
With today’s technology at our fingertips, I don’t believe there is a limit to how high we can build. We’ve beaten so many physical, scientific and technological restrictions over the past 4,000 years that with strong enough materials, a wide enough base, solid enough ground – and, I suppose, enough money – I see no reason why we can’t go as high as we want. The real question is: how high do we want to go? A wide base would probably mean very little daylight in the middle of the vast floors. Strong large columns and beams could mean restricted spaces in which to live and work. And what about the safety and convenience of the inhabitants: how long would you need to wait for an elevator, and how would you evacuate tens of thousands of people from a mammoth building?
Technology can undoubtedly take us there. New super-strong materials like graphene are already being synthesised in labs; cranes are getting larger; and new techniques like top-down construction are constantly being used in inventive ways. Science and engineering are leading to the creation of the mega-skyscraper – the Wuhan Greenland Centre (636m) in Wuhan, China; the Merdeka Tower (682m) in Kuala Lumpur, Malaysia; and the dart-like Jeddah Tower in Saudi Arabia, which will be the world’s first building to reach a height of 1km – at an unprecedented pace.
But where does it all stop?
The highest I’ve lived is on the 10th storey, and I loved the view and the new perspective of the city in which I lived. But I wonder how I would feel living much higher than that. In cities like Hong Kong or Shanghai, living on the 40th floor is common for thousands of people: it’s something the residents are used to. Eventually, perhaps, it will be commonplace everywhere: people are moving to cities in droves, and building high is a good way to fit all of us into an increasingly limited space.
The rapid growth in the height of buildings in the last century has barely given us a moment to consider if we likebeing so high above ground. But now, rather than racing ever higher, we are now stopping to think about our desires. It’s about what we want to build, not what we can. After a spate of building high towers from the 1960s to the 1980s, architects and engineers are questioning what type of buildings are really best for people and the environment. Cultural factors also play a part: different countries are at different stages in their urban development, and can have very different views about whether onwards and upwards is the best approach. I believe that, at some point in the future, the average height of our towers will plateau. Sure, iconic towers will still be built and they will continue to break records. Ultimately, however, our humanity will hold us back from the mega-tall. We want to live with sunlight and air flowing into our homes, and a connection to the earth and to our roots. We might gaze upwards at our structures and marvel at them, but we also need to feel grounded.
EARTH
Mexico City is built on a lake.
It started off as a small island but gradually expanded. The city now spreads far beyond its original site, but the centre of town, which contains most of the historical Aztec and Spanish buildings, sits on that lake. Twenty-eight metres down, the earth is strong and solid; everything on top of that is loose soil that was added later, and the result is very soft, very wet and very weak. It was described to me as a ‘bowl of jelly with buildings on top’.
Mexico City, which is built over a lake.
And so the historical centre of Mexico City is sinking. Fast. In the past 150 years it has subsided by over 10m – that’s more than a three-storey building.
*
When I was invited to Mexico to give a talk about my career and designing tall buildings, I jumped at the chance, not least because there was so much I wanted to see: the National Museum of Anthropology, the Bosque de Chapultepec, the ancient pyramids at Teotihuacan, and of course the Torre Latinoamericana, once the tallest skyscraper in Mexico City and still one of the best places to appreciate the sheer sprawling vastness of the metropolis. Naturally, I was also keen to explore the unique ground that lies below the city, and the bizarre effect it has had on the buildings there.
In engineering, what lies beneath the surface is just as important as what we can see above it. After all, you can have a well-designed superstructure (the bit above ground) – but if it’s not supported by an equally well-designed, stable substructure (the bit below the ground); if the layers and condition of the soil being built on aren’t properly understood; if you don’t build correctly within that ground – then the structure won’t be stable. The end result could be the Leaning Tower of Pisa. (Not the reason I would want tourists flocking to one of my buildings.) Knowing that Mexico City has some of the most challenging ground conditions in the world for building on – plus seismic susceptibility for good measure – I figured my trip was a fantastic opportunity to hear directly from the experts how they keep the city standing straight.
The site of the city was determined by a vision. The Aztecs were told by their god Huitzilopochtli (the God of War and the Sun) that they must move from their highland plateau, and that their new capital must be located where they found an eagle with a snake in its beak sitting on top of a nopal cactus (an image that is now the emblem on the national flag). The Aztecs set off and, after searching for just over 250 years, they found the eagle their deity had foretold. The fact that it was sitting on a tiny island in the middle of Lake Texcoco didn’t seem to trouble them (although I can imagine the tribe’s engineers cursing under their breath as they surveyed their new, watery building site).
Tenochtitlan, which means ‘place of the nopal cactus’, was founded in 1325. In its heyday, it was a beautiful city with fertile gardens, canals and massive temples, and its rulers commanded vast swathes of land. To connect the island city to the mainland, the Aztecs built three large causeways by pushing wooden logs vertically into the lake, and then creating pathways on top with soil and clay. These causeways are now the main roads that run through the hi
storical centre of the modern city.
Piles holding up buildings in soft ground.
The logs are examples of piles. They come in various shapes and sizes but share a common principle: they are columns put deep into the ground to help support the structure above them. If the ground is soft and not strong enough to support the weight of the structure, piles work to channel that weight in such a way that the soil is not overwhelmed. The ancients generally used tree trunks, but modern piles supporting larger structures are usually made from concrete shaped into cylinders, and sometimes from steel, cast in circular tubes, H or trapezoidal shapes. The foundations of the structure are built at the top of these piles and connected to them through steel bars.
Piles can channel forces into the ground in two ways: by means of friction between the surface of the piles and the soil, or by dumping forces at their base (‘end-bearing piles’). Depending on the weight and type of structure being supported, you can have multiple piles, which can vary in length depending on the forces they feel and the type of ground they engage.
Friction piles exploit the friction between the surface of a pile and the ground to carry the load or weight coming from the structure. The more piles you have, the more surface area is in contact with the ground, and the more friction is created. This friction force resists weight – thinking about it in terms of Newton’s Third Law, it is an upward reaction to the downward action of the superstructure.
Sometimes the ground is too loose to create friction against a pile, and then end-bearing piles are used. These are made long enough so that they poke into a deeper, stronger layer of ground. The load in the piles flows into their bases and dissipates into the earth.
In fact, piles don’t have to be either friction piles or end-bearing ones: they can be both. Some soils, such as clay, have good friction capacity because they bond to the pile. But say the load is so large, and you’re so restricted by available space, that friction alone isn’t enough to resist it. In that case you can make the piles long enough to reach a stronger layer of ground. In London, for example, there is a highly compact layer of sand approximately 50m deep that we drill down to for larger structures.
Working out how many piles to use, and how big to make them, is an important part of the engineer’s job. The starting point is the soil-investigation report, which tells me what the different layers of ground are, and how thick and strong they are. Then, if I find that a ‘pad’ of concrete will not be enough to stop the structure sinking, I’ll choose to use piles. By consulting the information in the report – and geotechnical engineers – I can calculate how deep the pile needs to be to hit a strong layer, and what the friction properties of the various layers are. I then have to decide on diameter. A small-diameter pile has the benefit of being cheaper and easier to install, but it may not be strong enough for the job. A larger-diameter pile has a bigger surface area, which increases the amount of friction; the area of the base is also bigger, making it stronger. The calculation is a search for the right compromise. I choose a diameter, calculate how much load a single pile will take based on a chosen length, then divide the total weight of the building by the capacity of one pile to work out how many piles I need. If I can fit that number of piles below the structure, then we can go ahead. If not, I make the pile bigger and repeat the calculation. For a 40-storey tower I designed near Old Street in London, we arrived at a total of about 40 piles between 0.6m and 0.9m in diameter, with some more than 50m long where the loads were greatest. Many modern skyscrapers are held up by piles that work by friction alone (if the ground is good enough so the piles can carry the loads they need to). But the piles in this tower work both by friction and by using end-bearing, as London’s clay is relatively weak to quite a depth.
Putting piles in the ground is a big challenge in itself. It wasn’t really until modern mechanisation that the huge piles we can now install were possible. Now, piles are often built using a sort of giant corkscrew that twists deep into the ground then reverses out, bringing the soil with it, and leaving a hole that is later filled with concrete. While the concrete is still wet, a steel cage is plunged in to reinforce the pile. For centuries, before mechanisation, most engineers simply pushed piles into the ground, as the Aztecs did at Lake Texcoco. From an engineering point of view their construction was successful, standing firm for the next two centuries.
But then the foreigners arrived.
The Spanish captured Tenochtitlan in 1521, razed it to the ground, and then rebuilt the city on the foundations of the Aztec pyramid temples. They cut down trees around the lake, causing mud slides and erosion that made the lake bed shallower. The water levels rose and the city flooded frequently throughout the seventeenth and eighteenth centuries, causing chaos and devastation (after the flood of 1629 the city was underwater for five years). Eventually, the lake was filled with soil to allow the city to expand, but it still suffered regular flooding because of the high level of water naturally present in the ground.
There is a level in the ground below which natural water flows and saturates the earth: this is known as the water table. Dig a hole in an area where the water table is high, and you’ll find that the hole fills with water pretty quickly: this is like the original Lake Texcoco. If you fill the hole with earth – which is like Lake Texcoco being filled with soil – then sprinkle on water to simulate rain, eventually water will puddle above the soil (just as our gardens are covered with puddles after a storm because the soil is saturated). This is what happened in Mexico City. The lake was filled in with soil but the water had nowhere to go. Then, the moment it rained, the rain added to the underlying water table and stagnated in the streets of Mexico City. It wasn’t until the twentieth century that the flooding was controlled using a huge network of tunnels that led the extra water away. But the legacy of building on such unpredictable, unstable ground can still be seen in the modern city.
*
Standing in the courtyard outside Mexico City’s enormous and very grey Metropolitan Cathedral, I scanned the crowds for Dr Efraín Ovando-Shelley, a geotechnical engineer who, according to his photo, wore sunglasses and khakis that made him look a bit like Indiana Jones. The solid, ordered columns of the cathedral were in sharp contrast to the delicate carvings between them, but what really caught my engineer’s eye were the cracks in the building. I could see where black space had opened up in the mortar and stone bricks, and the two huge bell towers that flanked the main entrance didn’t seem to be completely vertical. But such considerations were cut short when, at exactly the appointed time, Dr Ovando-Shelley appeared wearing his sunglasses, greeted me, handed over a book he had written, and led me towards the cathedral for a very unusual guided tour.
Metropolitan Cathedral, Mexico City.
As soon as we stepped through the entrance, (Map, point A) something felt very odd to me. Swarms of tourists stood rapt by the grandeur of the place, while worshippers sat respectfully hunched in its polished wood pews. But my attention was drawn to the floor. As we moved towards the back of the cathedral, I felt like I was walking uphill. And I was – because of the uneven or ‘differential’ settling of the ground that has taken place through history, the floor of the cathedral slopes upwards.
Map of the Metropolitan Cathedral.
Construction of the cathedral began in 1573, on top of the foundations of an Aztec pyramid. The architect, Claudio de Arciniega, knew of the problems with the ground and designed a clever foundation to deal with them. He started by driving more than 22,000 wooden stakes – each 3m to 4m long – into the ground, to ‘pin’ the soil together and compact it. Imagine a box of sand with lots of kebab skewers pushed into it in a grid pattern. If you shake the box, you’ll find that the sand moves around far less than if the skewers aren’t there. The stakes performed a slightly different function from piles, since they weren’t designed to take the weight of the cathedral, but rather to strengthen the soil.
Following this, the builders erected a massive masonry platform abov
e the stakes. It measured 140m by 70m – about the same width as a soccer pitch but one and a half times longer – and was about 900mm thick. Huge beams were laid on top of this platform in a grid pattern – a bit like a waffle – in such a way that the columns and walls of the cathedral could sit on top of them. The tops of the beams would eventually form the floor of the cathedral, spreading the weight of the columns onto the masonry platform, which in turn would spread the weight over the ground. This sort of foundation (with or without the large beams) is known as a ‘raft’ foundation.
The layers that form the raft foundation of the cathedral.
It does what its name suggests, which is to ‘float’ on top of the ground. When building on soft ground, the key is not to put large concentrated loads on the soil. If you do, it’s like standing on mud in stiletto heels. As many summer wedding guests will know, a sharp heel sinks into the ground because the pressure it exerts on the ground (calculated by dividing force by area) is high. Flat shoes, however, don’t sink as easily because the same force is spread over a much larger area – the snowshoe is based on this principle. So the masonry platform in the cathedral acted like a flat shoe on top of mud, spreading the weight of the building over a large area. The trouble, however, is that sometimes the ground is so soft that even spreading the weight of a structure across a large area, and avoiding concentrated loads, is not enough.
It’s probably worth noting here that friction or end-bearing piles were not used to support the weight of the structure. Perhaps because of the pyramid foundations below it, or perhaps because the engineers of the time realised that anchoring piles to the solid layer of earth might cause the opposite problem, making the cathedral rise. In fact, the Angel of Independence victory column in Mexico City (built in 1910) is supported on piles, and in the 100 years that have passed since it was built, 14 steps have been added to its base as it has become taller relative to its surroundings. Engineers in Mexico City agree that it’s best to allow the city’s structures to slowly, steadily and uniformly sink.
