by Roma Agrawal
The third effect is similar to a boat rocking at sea. Like trees, all buildings sway back and forth in the wind, depending on how strongly it is blowing – this is normal and safe. Unlike trees, however, buildings don’t move so much that you can easily see the displacement. Towers are generally designed to bend through a maximum distance of their height divided by 500 – so a 500m-tall tower won’t move more than 1m; but if this sway happens too quickly it could make you feel seasick.
One way to prevent a structure from toppling over is to make it heavy enough. In the past, most buildings were relatively modest in height and, because they were made from stone or brick, contained enough weight to resist the threat of the wind. But the higher you build, the stronger the wind is that you encounter. In the twentieth century, as we began to build taller and lighter structures, the force of the wind became a force to be reckoned with.
And so, in the modern skyscraper, weight alone is not always enough to keep it upright. Instead, the engineer must find a way to make the structure stiff enough to resist the wind. If you’ve ever watched a tree bending in a high wind and seen how it’s able to withstand such a force, then you already understand the principle engineers use to keep modern buildings upright, even if it’s blowing a gale outside. Just as a tree’s stability depends on a solid, well-rooted but pliable trunk, so a building’s stability often depends on a core, made from steel or concrete.
The core of a building, whether it’s concrete or steel, is designed to provide the stable ‘trunk’ of the structure and so must be well-rooted in the ground.
The core – which, as the name suggests, tends to be in the centre of a tower – is an arrangement of walls in a square or rectangle that extends vertically throughout the height of a tower – like the spine in the human body. The floors of the building are joined to the core walls. The reason we don’t generally notice cores is because they are well hidden, and usually themselves hide the essential services that are needed, like elevators, stairs, air ventilation ducts, electricity cables and water pipes.
Arranging the core of a building, usually hidden within the centre of the structure, which in turn provides a suitable place for essential services.
When wind hits the building, its force is channelled into and through the core. A building’s core is a cantilever – a structure, like a diving board, that is clamped firmly at one end and free to move at the other. The core is designed to flex a little and allow the wind forces to flow down into the foundations, stabilising the core and the building – much as a tree’s roots help it to withstand and disperse the wind’s power.
The walls in a concrete core will be made of solid concrete (apart from holes in specific places for elevator or stair doors), making the core inherently stiff. Steel cores are different: simply replacing concrete walls with steel ones would be incredibly expensive and heavy; the sheer weight of the steel would make them impossible to build. So instead of solid walls, steel columns and beams are arranged in formations of triangles and rectangles to create a frame or vertical truss.
The force in each steel section or concrete wall depends on which direction the wind is blowing. My computer model has the wind force values for 24 different directions from the wind tunnel report. The forces create compression and tension in the beams, columns and struts that make up the frame in a steel core, or the walls in a concrete one. The computer then works out the compression and tension in every bit of the core, for every orientation. We then design each steel section or concrete wall using the largest compression and tension figures. We vary the size of the steel, or the thickness of the concrete, depending on the force in each. The core thus keeps the tower stable irrespective of wind direction. It’s a complicated procedure to check the force in just one area for 24 different wind effects, let alone an entire core. Fortunately, computing power nowadays does the hard work, making it somewhat simpler for the engineer.
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The building at 30 St Mary Axe in London, which is 41 storeys tall and shaped like a gherkin (hence its nickname), has a different way of remaining stable in the face of wind. The elegantly curved cylinder of shaded blue glass is surrounded by large criss-crossing pieces of steel in the shape of big diamonds.
Completed in 2012, 30 St Mary Axe, London – otherwise known as ‘the Gherkin’ – has a steel exoskeleton to protect it from external forces.
A core is like a spine or a skeleton, giving a building integrity from the inside, but 30 St Mary Axe is surrounded by an exoskeleton. This exoskeleton – or, to use the technical term, external braced frame or diagrid – is like the shell of a turtle. Instead of an internal structure that resists the forces trying to push it over, it’s the shell or frame around the building that does the protecting. As it is buffeted by wind, the network of steel that forms the diagrid transmits the wind force to the foundations to keep the building stable.
Another spectacular example of the external braced frame is the Centre Pompidou in Paris. Architects Renzo Piano, Richard Rogers and Gianfranco Franchini envisioned what is, in effect, an inside-out building. All its arteries – the stuff that’s usually hidden away, like fresh-water and waste pipes, electricity cables, ventilation ducts, and even the stairs, elevators and escalators – are on the outside of the building. It’s these details that catch the eye and which people remember: the snaking pipes painted white, blue or green; the translucent tube of the escalator zigzagging upwards. But take a second look and you’ll notice that the whole structure is clad in a network of large X-shaped rods, which are there to keep it stable against the wind. An exoskeleton, among the air ducts and waste pipes.
The Pompidou Centre, Paris, has an external braced frame composed of a network of steel rods.
As a structural engineer, I like that I can see how the building works, and understand where the loads are going. Instead of hiding or disguising all the seemingly unglamorous but essential systems that make a building run smoothly, exposed systems like the Centre Pompidou’s are delightfully honest, and treat us to an insight into the character of a structure.
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Diagrids and cores, however, are not incorporated into buildings just to stop them toppling over – they also control sway. It might seem strange that our seemingly solid structures, made from steel and concrete, move – but they do. The swaying in itself is not a problem: what’s important is how fast the building sways, and for how long. Through years of experiments we’ve been able to determine the levels of acceleration (a measure of how quickly the speed of an object is changing) at which humans can feel this movement. Take travelling in an aeroplane, for example: even though it flies extremely fast, in calm air you hardly feel you’re moving at all. When you hit turbulence, however, the speed starts to change suddenly and quickly, and you feel it. Buildings are similar: they can move by quite a large amount, and you won’t feel anything so long as the acceleration is small. But if the acceleration is large, then even if the building is only moving a small amount you could feel queasy.
It’s not just the acceleration that affects us. How long the building continues to sway – how long it oscillates or moves side-to-side – can also make us feel unsteady. To use a diving-board analogy once more: when you bounce on the board and take a dive, the board oscillates before it stops moving. A thick board that is strongly clamped at its end only oscillates a short distance and stops after just a few oscillations. A thinner, weaker board that isn’t as strongly clamped will oscillate a greater distance and for a longer time.
When I design a tall tower, I have to make sure that the acceleration of the sway is outside the range of human perception, and that the oscillation stops quickly.
The same computer model that helps me design a structure that can resist gravity and wind also helps me with this challenge. I enter the materials, shape and size of the beams, columns and core into the programme. The software then analyses the wind force, the materials’ stiffness and the geometry of the structure, and tells me what the acceleration is. If
it is below the threshold that people can feel, then nothing more needs to be done. If, however the acceleration is greater, then I need to make the structure stiffer. We can achieve this by increasing the thickness of the walls of a concrete core, or using bigger steel struts in a steel one. I then rerun the model, sometimes many times, until the target acceleration is reached.
The taller and more slender the tower, the more pronounced the sway. Sometimes it isn’t possible to stiffen the structure enough to control the acceleration and how long it oscillates. So although the building is perfectly safe, it wouldn’t feel safe. In that case, the sway of the tower is artificially controlled using a form of pendulum called a tuned mass damper, which moves in the opposite direction to the tower.
Every object, including buildings, has a natural frequency: the number of times it vibrates in one second when it is disturbed. An opera singer can shatter a wine glass because the glass has its own natural frequency. If the singer can hit a note with the same frequency as the glass, the energy of her voice causes the glass to vibrate dramatically until it rips itself apart. Similarly, wind (and earthquakes) can shake buildings at a particular frequency. If the natural frequency of the building is the same as that of the gusts of wind or the earthquake, the building will vibrate dramatically, and will be damaged. This phenomenon – an object vibrating dramatically at its natural frequency – is called resonance.
A pendulum – which is basically a weight suspended by cables or springs – oscillates back and forth. Depending on the length of the cable, or the stiffness of the springs, it swings a fixed number of times in a fixed period. When using a pendulum to cancel out a skyscraper’s sway, the trick is to calculate the skyscraper’s frequency (using a computer model), and then to install a pendulum with a similar frequency at the top.When wind or an earthquake hits the skyscraper, it starts to move back and forth. This causes the pendulum to oscillate as well – but in the opposite direction to the tower.
A pendulum cancels out the sway of a tall building by swinging in the opposite direction.
You can stop the vibration of a tuning fork – and therefore its sound – just by touching one of its prongs. Your finger absorbs the energy of the vibration. The same process is at work in our swaying skyscraper. The building is like the tuning fork and the pendulum acts like your finger, absorbing the energy created by the movement of the skyscraper, which moves less and less. The movement of the structure is said to be ‘damped’ (hence the term ‘tuned mass damper’), so the people inside can’t feel it.
Taipei 101, the 509m tower in Taipei City in Taiwan, was the tallest building in the world when it was completed in 2004. It is deservedly famous for its distinct architectural aesthetic: inspired by pagodas and stalks of bamboo, the building is composed of eight trapezoidal sections that give it a ridged, organic feel, as though it has pushed its way out of the ground like the stem of a plant – an illusion reinforced by the tinted windows, which give it a green hue.
Standing at 509 metres tall, the Taipei 101 tower dominates the skyline of Taipei City, Taiwan.
But the tower is also famous for the huge ball of steel that hangs between the 92nd and 87th floors. At 660 tonnes, this steel pendulum is the heaviest in any skyscraper in the world. It is a huge tourist attraction (its sheer scale, geometrical elegance and bright yellow colour make it look like something from a sci-fi film), but its real purpose is to protect the tower from the typhoons and earthquakes that can hit the city. When the building is shaken by a storm, or by an earthquake vibrating the ground beneath it, the pendulum swings into action, oscillating to absorb the movement of the tower. In August 2015, Typhoon Soudelor swept across Taiwan, gusting to at least 170km an hour, but Taipei 101 escaped undamaged. Its saviour, the pendulum, recorded movement of up to 1m – its largest ever movement.
The pendulum in Taipei 101 is how the building survives earthquakes.
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Engineers use a pendulum to defend against wind and earthquakes because both are random forces that act in a horizontal direction. But earthquakes can have far more devastating effects, so we often need other precautions too. The terrifying, annihilative power of the earthquake gave rise to all manner of explanations for its origins. Ancient Indian mythology says that the Earth shakes when the four elephants that carry it on their backs move or stretch. According to Norse myths, the Earth trembles when Loki (the God of Mischief, imprisoned in a cave for his misdeeds) wrestles with his restraints. The Japanese blame the giant catfish, Namazu, which lives underneath the Earth in mud, guarded by a god who holds it down with a huge stone. Sometimes, however, the god becomes distracted and allows Namazu to thrash about. Nowadays we have a less colourful but more accurate explanation for the periodic vibration of the Earth. Earthquakes happen when different layers of the Earth’s crust move relative to one another. A wave of energy explodes from a single point: the epicentre. The energy spreads outward from this point, shaking everything on the surface, including our structures. The waves of energy from the tremors that affect our structures are unpredictable and irregular – they strike without warning.
Engineers study the frequencies of earthquakes in historical records, then they use a computer model to compare these to the natural frequency of the building to be constructed. Just like we did for wind, we must ensure that the two frequencies aren’t too similar, otherwise the building will resonate and could be damaged, or even collapse. If they are, the natural frequency of the building can be changed by adding more weight to it, or by making the core or frame of the structure stiffer.
Another way to mitigate the effects of an earthquake’s energy waves is to use special rubber ‘feet’ or ‘bearings’. If you sit in your living room with powerful speakers busting out some bass, you feel vibrations transmit from the speakers, into the floors, through the sofa and finally into your body. Put some rubber feet on the underside of the speakers and the effect lessens, because the feet absorb most of the vibrations. Similarly, we can install big rubber bearings at the bottom of the columns of a building, which then absorb an earthquake’s vibrations.
Dampers protecting the Torre Mayor skyscraper, Mexico City, Mexico.
Earthquake energy can also be absorbed in the connections between beams, columns and diagonal braces. The Torre Mayor skyscraper in Mexico City employs a very clever system to do this. In this 55-storey structure, 96 hydraulic dampers or shock absorbers – like pistons in a car – are arranged in X shapes all around the building and across its full height (creating a diagrid) to act as extra bracing against earthquakes. When an earthquake occurs, the whole building sways and the movement is absorbed into these dampers so the structure itself doesn’t move too much. In fact, very soon after the Torre Mayor was completed, an earthquake recorded at a magnitude of 7.6 shook Mexico City, causing widespread damage. The Torre Mayor building survived unscathed; it’s said that the occupants did not even realise there had been an earthquake.
And this, in a way, is the engineer’s ideal – a building so well-designed, and so secure, that its occupants carry on comfortably with their business, completely unaware of the amount of complicated technology tackling all the forces the structure has to withstand each day.
FIRE
On the morning of 12 March 1993, I went to school in the Juhu district of Mumbai as usual, with my hair tied neatly back, wearing a crisp white blouse and grey pinafore. My teeth were hidden by braces, which were interwoven with my choice of green bands; definitely not cool (yes, even at nine I was already the class nerd). At 2.00pm Mum picked up my sister and me in our lime-green Fiat and took us home. While she was parking the car, we raced up four flights of stairs in our daily competition to see who could make it to our front door first. But something felt different. We stopped at the last step; we couldn’t get to the door because our neighbour was standing there, nervously fiddling with her dupatta, looking distressed.
We soon discovered why. While Mum was collecting us from school there had been a bomb attack on
the Bombay Stock Exchange – the building where my father and uncle worked.
Panicking, we ran into the flat and switched on the television. Every news channel was covering the mayhem. Bombs continued to explode around the city. Hundreds had been killed and injured. This was before the advent of mobile phones, so we had no way of knowing if my father and uncle were alive and safe.
The Bombay Stock Exchange is a 29-storey concrete tower in the heart of Mumbai’s financial district. A car carrying a bomb had made its way into the basement garage and then detonated. Many lives were lost; many more people were hurt. I stood in front of the television horrified, watching images of weeping people covered in blood and dust running from billowing smoke. Police cars, fire trucks and ambulances raced to the tower, sirens blaring. We could see that the offices on the ground and first floor nearest to the explosion had been destroyed. It was clear that no one in that part of the building could have survived. Dazed people from the higher floors clambered down stairs and out of the tower. At home, we looked at each other and didn’t utter a word, but I knew the same thought was running through all our minds. My dad and uncle worked on the eighth floor. We quietly hoped for the best.
