by Roma Agrawal
The panel of judges tasked with choosing the dome’s final design repeatedly asked him to reveal his methods, but Brunelleschi refused. At one of the judging meetings, where a number of experts were present and also bidding for the commission, he asked for an egg to be brought into the room. If any of his rivals could make the egg stand on its end, he said, they should win the competition. One by one people took the challenge, and failed. Brunelleschi then tapped the egg hard on the table and left it standing where it was (with a partially broken shell). When the others protested that anyone could have done that, had they known they could break the shell, he countered: ‘Yes, and you’d say the same thing if I told you how I intend to build the dome.’ He won the contract – though possibly only because there were few other practical solutions. (One person had even suggested filling the cathedral with earth to support the dome during construction. After the dome was completed, the earth would be cleared by small boys eager to get hold of coins deliberately mixed in at the outset.)
I visited Florence when I was a physics student. With the Ponte Vecchio, Giotto’s Campanile, the Baptistery and Santa Felicita, it’s like an open-air museum of medieval and early Renaissance engineering. Il Duomo, as the city’s cathedral is affectionately known, is of course one of its centrepieces. I stood outside for a while, taking it all in – the neat symmetry of its three doorways, separated by four tall columns (with another two up above), and a series of very intricate carvings of Mary and the apostles just below the largest of the rose windows. Circles, pointed arches, triangles and rectangles, with coloured bands of stone, came together in pleasing geometric chaos. Eventually I passed through the doorway and my eyes were immediately drawn to the underside of the dome, high above me.
The base was an octagon, and each side had a circular stained-glass window letting in shafts of light. More light entered through an oculus at the top of the dome. Above the stained-glass windows were spectacular frescoes depicting The Last Judgement – choirs of angels, saints and personifications of the virtues vied for attention amid layers of painted cloud. It was all lovely, but the scientist in me wanted to know how it worked, to see the dome behind its beautiful embellishments.
The best view of the dome is from Giotto’s bell tower, which stands in the piazza near the western corner of the cathedral. The 414 stone steps tested my fitness, but eventually I found myself at the top, looking out at the bank of deep red terracotta tiles and a few of the eight white ribs that define the dome’s shape. It was a thrilling viewpoint, and a fitting tribute to Brunelleschi’s genius. For me, it’s Brunelleschi’s unconventional thinking, coupled with the courage to make it a reality, that makes him relevant to modern engineering. It’s by thinking beyond the orthodoxy and imagining the ‘impossible’ that we move engineering forward.
The skeleton of the Duomo that lies between the two layers of brickwork, Brunelleschi’s innovation.
Brunelleschi drew the ribs in characteristically detailed sketches. The ribs were made from stone, acting as arches that landed on the eight corners of the hole. These arches supported the edges of the octagonal dome. Between the main eight stone ribs were a further sixteen designed to resist the force of the wind. I couldn’t see these from the outside, because Brunelleschi hid them away in the hollow space between two layers of brick skin. By creating this hollow space, not only was he able to hide the secondary ribs, he could also reduce the weight of the dome to half of what it would have been if it was solid. This reduction in weight helped him build the dome without centering.
Brunelleschi had gone back to basics. Brick structures are traditionally built in layers, comprising brick, then a layer of mortar, then another layer of brick, and so on. Imagine a simple garden wall and you’ve got the idea. Say, however, that you need this wall to curve in towards you (unlikely, I know, but bear with me). At that point, the problems begin: as the wall curves and becomes taller and heavier, it’s in danger of overloading and cracking. Mortar is usually weaker than brick, so the continuous layer of mortar, rather than the bricks, is most likely to fail first.
To counter this, Brunelleschi asked his bricklayers to do something they had never done before. He directed them to lay three bricks horizontally, and then to place bricks vertically, like bookends, at either side of the horizontal group. The next layer again alternated three horizontal bricks with vertical bricks at each end. It was a painstaking process: four million bricks were laid; workers patiently waited for the mortar to dry on one layer before they started on the next. The layers created a ‘herringbone’ pattern, so-called because it supposedly looks like the bones of a fish. As an engineer, I admire this idea because of its simplicity. Since continuous lines of mortar were the weak link, Brunelleschi broke up the lines with vertical bricks, making the curving wall far stronger.
A herringbone brick-laying formation in which the vertically laid bricks add strength.
A similarly innovative approach drove the construction of The Shard. While designing its spine (or core), the team of engineers I worked with devised a unique method to build it. To save time on the construction programme, we decided to work in two directions: digging down to form the basement and at the same time constructing upwards. Usually when you want to make a basement, you dig an immense hole with concrete or steel walls holding up its sides. Piles – long shafts of concrete – are installed at the bottom of the hole to support the future building. Then slabs are poured at each basement storey until you get back up to ground level. It’s only at this point that anything can be built above ground.
But we did something unprecedented. We asked for the piles to be installed at ground level, and huge steel columns to be plunged into the piles. First, the ground floor slab was built, with a giant hole in it. This hole gave workers access to the soil, then diggers removed earth to expose the concrete piles with steel columns inside them. While digging continued downward, a special rig was attached to the newly exposed steel plunge columns, this rig could build the central concrete core. As the core rose, the basement and foundations were finished. At one point, twenty floors of the huge concrete spine were being held up just by the steel columns – there was no foundation in place. It was a structure on stilts.
The top-down construction method, which was employed during the building of The Shard, London.
This method, called ‘top-down’ construction, had been used previously to hold up columns and floors in small structures. But it had never been used on a core, let alone one of this size. It was an engineering first. Our ability to think beyond standard practice saved time and money – we solved a real-world challenge with creativity. Others are now using our idea in their projects – as always, building on existing ideas leads to innovation, whether it’s in one of the most famous cathedral domes in the world, or one of the tallest buildings in Europe.
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On that site visit to The Shard in May 2012, as I shot up the tower in my cage-like hoist to the 34th and then the 69th floors, my eyes glued to the building rather than looking out and down, I couldn’t help reflecting on how, without elevators, The Shard – indeed, any skyscraper – simply wouldn’t exist. Part of the reason Roman insulae stopped at ten storeys was because climbing up and down any further was impractical. Today, we’re so used to pressing a button and summoning a mobile cubicle to whisk us up and down our multi-storey towers that we don’t give it a second thought. But before the 1850s, elevators in this form didn’t exist. And although we started to build skyscrapers fairly soon after the invention of the elevator, such a device wasn’t originally designed with buildings in mind, but as a safer way to move materials around a factory.
Like Archimedes, Elisha Otis had a restless and creative imagination. While working in a variety of jobs – carpenter, mechanic, bedstead manufacturer, factory owner – he invented an automatic turner that made the production of bedsteads four times faster; a new type of railway safety brake; and even an automatic bread-baking oven. In 1852 he was hired to clear a factory
in Yonkers, New York and, frustrated by the effort involved in transporting materials between floors manually, he turned his attention to how best to accomplish the job mechanically. Methods for moving people and materials from one storey to another had been around for centuries: Roman gladiators, for example, rose from the pits of the Colosseum up into the fighting arena on a moving platform. The problem, however, was that they weren’t safe: if the rope shifting the platform up or down suddenly snapped, the platform fell to the ground, probably killing its occupants. Otis wondered if he could fashion something that would prevent this from happening.
The wagon spring solved the challenges of operating an elevator.
His idea was to make use of the ‘wagon spring’: a C-shaped spring made up of carefully layered thin steel strips that was commonly used to improve the suspension in carriages and wagons. When it has force on it, a wagon spring is almost flat, but when it’s released, it curves. It was this change of shape, caused by force, that Otis planned to use to his advantage. First, he replaced the smooth guide rails (which kept the platform in position during its progress up and down) with toothed or ratcheted rails. Then he created a mechanism in the shape of a goalpost, which had a hinge in the middle and feet sticking out at the base. He attached the spring, then the goalpost, to the rope at the top of the elevator car. When the rope was intact, the spring remained flat and the goalpost square. If the rope was cut, the spring sprung into a C-shape, pushing down on the goalpost and deforming it so that its two ‘feet’ stuck into the ratcheted rails, bringing the elevator to a halt.
This diagram is included in the patent documents for the Otis Elevator – or ‘hoisting apparatus’.
But to bring his invention to the attention of the public, and show them that it worked, Otis needed a big stage – and he found it at the 1853 World’s Fair in New York. Entitled the ‘Exhibition of the Industry of All Nations’, the exposition aimed to show off American technological might, and showcase industrial innovation from around the world. In the vast exhibition hall Otis constructed his elevator with guide rails, ratchets, springs, platform and hoisting machinery, and loaded the platform with goods. When a crowd had gathered, he climbed on top of the platform and had it lifted to its maximum height. As the crowd looked on, he called for the hoisting-rope to be cut, and his assistant swung the axe.
There were gasps as the platform suddenly lurched downwards. And then, just as suddenly, it stopped. It had fallen only a few inches. From the top of it Otis could be heard shouting, ‘All safe, gentlemen. All safe.’
Four years later, Otis installed his first, steam-powered safety elevator in the five-storey E.V. Haughwout & Co. department store on the corner of Broadway and Broome Street in New York. The eponymous company he founded has continued to supply elevators and escalators to buildings around the world, from the Eiffel Tower and Empire State Building to the Petronas Towers in Malaysia. Such buildings would hardly have been possible without Otis’s invention. Until he developed the safety elevator, the height of a building was restricted by how many stairs people were prepared to climb. The elevator smashed that barrier and engineers could start to think about true skyscrapers. Since then we’ve been building higher and higher, and we now have the opposite problem: we can’t make elevators that travel much further than 500m because the steel cables to hoist them up and down become too heavy for the machinery to work efficiently. It’s one reason why elevators often don’t go all the way to the top of very tall towers. You go up a number of floors, then change elevators to go up the rest. But engineers are already exploring ways to solve this by using different materials. Replacing steel with carbon fibre – which is stronger but lighter – seems one way forward, but questions remain about how well the carbon fibres can resist fire. As our towers continue to grow, these innovations will be much needed.
Another challenge in super-tall towers is sway. In chapter 1 I talked about controlling the movement of buildings to stop us feeling sick. But there is another reason this control is needed. Elevators run on straight guide rails, and as towers move the elevator shafts and the guide rails fixed to them curve. A small amount of curve is not a problem – the cogs and clasps of the elevator car on the rails have a little give – but too much and the car will grind to a halt, unable to move. The taller buildings become, the more they move and the more curve you experience in the elevator shaft. There are solutions to the problem, ranging from upgrading the elevators themselves, to allowing more give, to stopping elevators running in the worst storms. Ultimately, I’m sure, a modern-day Otis will come up with an ingenious solution. And he – or she – will have to, because the elevator has become an intrinsic part of our everyday life. The equivalent of the entire world’s population is moved in an elevator every 72 hours.
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I was reminded of Elisha Otis during my visit to the Burj Khalifa in Dubai, the world’s tallest building (at 829.8m), because his company installed the elevators that were about to take me to the observation deck on the 124th of its 163 floors. It was a more serene journey than my trip up the outside of the tallest tower in Western Europe in a cage-like hoist, although the floor number on the LCD display changed with a bewildering rapidity as we ascended at 36km/h. (Elisha Otis’s original elevator in the E.V. Haughwout Building climbed at just over 0.7km/h.) A minute later I emerged to an unparalleled view. On one side, pure sand extended beyond the buildings to the horizon. On the other, I could see the blue sea and, far away to the left, the cluster of man-made islands that form the famous leaf shape of the Palm Jumeirah. Steeling myself, and feeling protected by the floor-to-ceiling glass, I ventured closer to the edge and looked down. Beneath me were a number of tiny, futuristic-looking buildings, like scale models on the set of a sci-fi film. It was a shock to realise that these structures are actually taller than most of the skyscrapers in Europe, and many even in the US. The Burj Khalifa dwarfs everything around it, and plays havoc with your sense of proportion.
Burj Khalifa in Dubai, the world’s tallest building in 2018, which has been made possible partly by the developments in elevator technology.
‘Megatall’ skyscrapers like the Burj Khalifa were made possible by a man who started life as a mischievous and lively-minded young boy, born in Dhaka, Bangladesh, in April 1929. Fazlur Khan disliked traditional schooling methods: his inquisitive questions were met with stern responses from teachers; as a result, he didn’t take education very seriously (even though his father was a mathematics teacher). Fortunately, his patient, forward-thinking dad realised that his son needed a broader education, and was determined to further his intellectual curiosity while fostering a sense of discipline. He set Fazlur problems similar to those in his school homework, but which made the boy consider solutions far beyond what the homework asked for; he also challenged him to solve the same problem from multiple perspectives. When the time came for Fazlur to choose whether to study physics or engineering at university, his father guided him towards the latter because, he said, it demanded discipline and would require him to wake early for lectures. (In fact, as I can attest, a physics degree involves a lot of early-morning lectures too.) Khan gained a degree in civil engineering at Dhaka University in 1951, finishing first in his class, and went to the US on a Fulbright Scholarship in 1952. In the next three years he acquired two master’s degrees and a PhD, while also learning French and German.
It was Khan who came up with the idea of putting a building’s stability system on the outside – a brilliant innovation that has since been used on iconic structures around the world, from the Centre Pompidou and the Gherkin to the Hearst and Tornado Towers. Using large pieces of diagonal bracing to form strong triangles, Khan created a stiff external skeleton, effectively turning traditional skyscrapers inside out. This system is often called a ‘tubular system’ because, like a hollow tube, the outside ‘skin’ of the structure gives it strength, although the shape of the skin doesn’t have to be cylindrical.
An alternative stability system for build
ings is to forgo the conventional central core and instead employ an exoskeleton.
Khan’s first commission to employ this concept was the DeWitt-Chestnut apartment building in Chicago. But the real showcase for his novel approach was the completion in 1968 of the city’s John Hancock Center which, at 100 storeys (344m), became the second-tallest skyscraper in the world after the Empire State Building. It is a rectangular cuboid with gently tapering faces, making it narrower at the top than at the base. On each face you can see five giant ‘Xs’, one on top of the other, that form the bracing for the tower. Fifty years on, its eye-catching design still looks modern and elegant. The pioneering design earned Khan the catchy title ‘father of tubular designs for skyscrapers’.
The John Hancock Center in Chicago utilises an exoskeleton to give the tower stability.
The external skeleton was only one of Khan’s ideas. He also suggested combining many such skeletons in a cluster. This is like holding a bunch of straws in your hand: each straw is a single tube which by itself is stable up to a certain point; by bunching lots of straws together, however, you can make a much stiffer and more stable structure. The Burj Khalifa employs a variation of this system. Look at a cross-section of the structure and you’ll see that it has a distinctive tripartite shape that resembles leaves or petals. (It’s become a kind of brand image for the building: as you ascend in the elevator, a lightshow of row upon row of the shapes dances across the walls in different configurations.) The ‘petals’ are in fact a series of ‘straws’ or tubes with exoskeletons of their own which – in their cluster – support one another. This mutual support between the individual pieces means that the tower remains stable despite being so high.
